diff --git a/auth/apikey.go b/auth/apikey.go
new file mode 100644
index 0000000..21e2891
--- /dev/null
+++ b/auth/apikey.go
@@ -0,0 +1,166 @@
+package auth
+
+import (
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
+	"encoding/hex"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"net/http"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// APIKey is the stored record for a minted key. Only the SHA-256 of the
+// full key string is kept; the key itself is shown exactly once at mint
+// time. SHA-256 (not argon2/bcrypt) is sufficient because keys are 192-bit
+// random secrets, not low-entropy passwords.
+type APIKey struct {
+	ID        string    `json:"id"`
+	Name      string    `json:"name"`
+	Hash      string    `json:"hash"` // hex sha256 of the full key string
+	Scopes    []string  `json:"scopes,omitempty"`
+	CreatedAt time.Time `json:"createdAt"`
+}
+
+// ErrKeyNotFound is returned by Revoke for an unknown key id.
+var ErrKeyNotFound = errors.New("auth: api key not found")
+
+// KeyStore is a file-backed set of API keys (<file> is JSON, written
+// atomically). Volume is tiny — a handful of integration keys — so a flat
+// file beats dragging key records into the event-sourced graph store.
+type KeyStore struct {
+	path string
+	mu   sync.Mutex
+	keys []APIKey
+}
+
+// OpenKeyStore loads (or initializes) the key store at path.
+func OpenKeyStore(path string) (*KeyStore, error) {
+	s := &KeyStore{path: path}
+	raw, err := os.ReadFile(path)
+	if errors.Is(err, os.ErrNotExist) {
+		return s, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	if err := json.Unmarshal(raw, &s.keys); err != nil {
+		return nil, fmt.Errorf("auth: parse %s: %w", path, err)
+	}
+	return s, nil
+}
+
+// Mint creates a key with the given display name and scopes, persists its
+// hash, and returns the full key string — the only time it is available.
+func (s *KeyStore) Mint(name string, scopes []string) (key string, rec APIKey, err error) {
+	idBytes := make([]byte, 4)
+	secret := make([]byte, 24)
+	if _, err = rand.Read(idBytes); err != nil {
+		return "", APIKey{}, err
+	}
+	if _, err = rand.Read(secret); err != nil {
+		return "", APIKey{}, err
+	}
+	id := hex.EncodeToString(idBytes)
+	key = "gck_" + id + "_" + base64.RawURLEncoding.EncodeToString(secret)
+	rec = APIKey{
+		ID:        id,
+		Name:      name,
+		Hash:      hashKey(key),
+		Scopes:    scopes,
+		CreatedAt: time.Now().UTC(),
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.keys = append(s.keys, rec)
+	if err := s.saveLocked(); err != nil {
+		s.keys = s.keys[:len(s.keys)-1]
+		return "", APIKey{}, err
+	}
+	return key, rec, nil
+}
+
+// Revoke deletes the key with the given id.
+func (s *KeyStore) Revoke(id string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i, k := range s.keys {
+		if k.ID == id {
+			s.keys = append(s.keys[:i], s.keys[i+1:]...)
+			return s.saveLocked()
+		}
+	}
+	return ErrKeyNotFound
+}
+
+// List returns the stored key records. Callers serving them over HTTP
+// should strip Hash themselves — it isn't secret-bearing, but is also not
+// a client concern.
+func (s *KeyStore) List() []APIKey {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	out := make([]APIKey, len(s.keys))
+	copy(out, s.keys)
+	return out
+}
+
+// lookup finds the record whose hash matches the presented key.
+func (s *KeyStore) lookup(presented string) (APIKey, bool) {
+	h := []byte(hashKey(presented))
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for _, k := range s.keys {
+		if subtle.ConstantTimeCompare(h, []byte(k.Hash)) == 1 {
+			return k, true
+		}
+	}
+	return APIKey{}, false
+}
+
+// saveLocked writes the store atomically (tmp + rename). Callers hold s.mu.
+func (s *KeyStore) saveLocked() error {
+	raw, err := json.MarshalIndent(s.keys, "", "  ")
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(s.path), 0o755); err != nil {
+		return err
+	}
+	tmp := s.path + ".tmp"
+	if err := os.WriteFile(tmp, raw, 0o600); err != nil {
+		return err
+	}
+	return os.Rename(tmp, s.path)
+}
+
+func hashKey(key string) string {
+	sum := sha256.Sum256([]byte(key))
+	return hex.EncodeToString(sum[:])
+}
+
+// APIKeyAuth authenticates requests carrying `X-API-Key: <key>` against a
+// KeyStore. The Principal's Subject is the key id and its Scopes are the
+// scopes the key was minted with.
+type APIKeyAuth struct {
+	Keys *KeyStore
+}
+
+var _ Authenticator = APIKeyAuth{}
+
+func (a APIKeyAuth) Authenticate(r *http.Request) (*Principal, error) {
+	presented := r.Header.Get("X-API-Key")
+	if presented == "" {
+		return nil, ErrNoCredentials
+	}
+	rec, ok := a.Keys.lookup(presented)
+	if !ok {
+		return nil, errors.New("auth: invalid api key")
+	}
+	return &Principal{Subject: rec.ID, Scopes: rec.Scopes, Kind: KindAPIKey}, nil
+}
diff --git a/auth/auth.go b/auth/auth.go
new file mode 100644
index 0000000..d8d47a4
--- /dev/null
+++ b/auth/auth.go
@@ -0,0 +1,87 @@
+// Package auth is the stable cross-service authentication seam for the
+// platform. It is provider-agnostic: one Authenticator interface with
+// interchangeable impls — OIDC (validates a bearer JWT against any OIDC
+// issuer; Zitadel is the default reference), API key (validates X-API-Key
+// against the KeyStore), and a Chain combinator that runs them in order.
+//
+// Packages under platform/ import only the standard library and other
+// platform/ packages — never an application module. Application modules
+// depend downward onto platform. This is what keeps the auth seam stable
+// and reusable.
+package auth
+
+import (
+	"errors"
+	"net/http"
+	"slices"
+)
+
+// Kind values for Principal.Kind. Comparing against these strings is the
+// authorization layer's primitive: handlers ask "is this OIDC or an API
+// key?" rather than re-implementing the chain logic.
+const (
+	KindOIDC      = "oidc"
+	KindAPIKey    = "apikey"
+	KindAnonymous = "anonymous"
+)
+
+// ErrNoCredentials is returned when a request carries no credentials for the
+// authenticator (no bearer token / no API key). Chain treats it like any
+// other failure and falls through to the next authenticator.
+var ErrNoCredentials = errors.New("auth: no credentials")
+
+// Principal is the authenticated caller. Authorization (scope checks) is a
+// thin layer on top: handlers ask "does this principal have scope X", not
+// "how did it authenticate".
+type Principal struct {
+	Subject string   `json:"subject"`          // OIDC sub or API key id
+	Scopes  []string `json:"scopes,omitempty"` // e.g. ["write","admin"]
+	Kind    string   `json:"kind"`             // KindOIDC | KindAPIKey | KindAnonymous
+}
+
+// HasScope reports whether the principal carries the given scope.
+func (p *Principal) HasScope(scope string) bool {
+	return slices.Contains(p.Scopes, scope)
+}
+
+// Authenticator inspects request credentials and returns the caller, or an
+// error if authentication fails.
+type Authenticator interface {
+	Authenticate(r *http.Request) (*Principal, error)
+}
+
+// Chain tries each authenticator in order, returning the first successful
+// Principal. It is how bearer-then-apikey composition is expressed.
+type Chain []Authenticator
+
+var _ Authenticator = Chain(nil)
+
+// Authenticate returns the first non-error result. On failure it returns
+// the first *real* validation error — an authenticator reporting
+// ErrNoCredentials merely didn't apply (e.g. no X-API-Key header alongside
+// a bad bearer token) and must not mask why the presented credential was
+// rejected.
+func (c Chain) Authenticate(r *http.Request) (*Principal, error) {
+	var firstErr error
+	for _, a := range c {
+		p, err := a.Authenticate(r)
+		if err == nil {
+			return p, nil
+		}
+		if firstErr == nil || (errors.Is(firstErr, ErrNoCredentials) && !errors.Is(err, ErrNoCredentials)) {
+			firstErr = err
+		}
+	}
+	return nil, firstErr
+}
+
+// Anonymous authenticates every request as an unauthenticated principal
+// with no scopes. It is the placeholder for tests and local-dev runs; it
+// must never be wired in production.
+type Anonymous struct{}
+
+var _ Authenticator = Anonymous{}
+
+func (Anonymous) Authenticate(*http.Request) (*Principal, error) {
+	return &Principal{Subject: "anonymous", Kind: KindAnonymous}, nil
+}
diff --git a/auth/auth_test.go b/auth/auth_test.go
new file mode 100644
index 0000000..0f1d0be
--- /dev/null
+++ b/auth/auth_test.go
@@ -0,0 +1,302 @@
+package auth
+
+import (
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+// --- OIDC ---
+
+// fakeIssuer hosts a discovery document and a JWKS for a generated RSA key,
+// and mints RS256 tokens signed by it.
+type fakeIssuer struct {
+	srv *httptest.Server
+	key *rsa.PrivateKey
+}
+
+func newFakeIssuer(t *testing.T) *fakeIssuer {
+	t.Helper()
+	key, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		t.Fatal(err)
+	}
+	f := &fakeIssuer{key: key}
+	mux := http.NewServeMux()
+	mux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, _ *http.Request) {
+		_ = json.NewEncoder(w).Encode(map[string]string{"jwks_uri": f.srv.URL + "/jwks"})
+	})
+	mux.HandleFunc("/jwks", func(w http.ResponseWriter, _ *http.Request) {
+		pub := &key.PublicKey
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"keys": []map[string]string{{
+				"kty": "RSA",
+				"kid": "test-key",
+				"n":   base64.RawURLEncoding.EncodeToString(pub.N.Bytes()),
+				"e":   base64.RawURLEncoding.EncodeToString(mustBigIntBytes(pub.E)),
+			}},
+		})
+	})
+	f.srv = httptest.NewServer(mux)
+	t.Cleanup(f.srv.Close)
+	return f
+}
+
+// mint signs an RS256 JWT with the given claims (kid "test-key").
+func (f *fakeIssuer) mint(t *testing.T, claims map[string]any) string {
+	t.Helper()
+	enc := func(v any) string {
+		b, err := json.Marshal(v)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return base64.RawURLEncoding.EncodeToString(b)
+	}
+	signing := enc(map[string]string{"alg": "RS256", "kid": "test-key"}) + "." + enc(claims)
+	digest := sha256.Sum256([]byte(signing))
+	sig, err := rsa.SignPKCS1v15(rand.Reader, f.key, crypto.SHA256, digest[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	return signing + "." + base64.RawURLEncoding.EncodeToString(sig)
+}
+
+// mustBigIntBytes turns an RSA exponent (typically 65537) into its
+// big-endian byte encoding for the JWK "e" field.
+func mustBigIntBytes(e int) []byte {
+	// smallest big-endian encoding of e.
+	n := e
+	if n == 0 {
+		return []byte{0}
+	}
+	var buf []byte
+	for n > 0 {
+		buf = append([]byte{byte(n & 0xff)}, buf...)
+		n >>= 8
+	}
+	return buf
+}
+
+func bearerReq(token string) *http.Request {
+	r := httptest.NewRequest("GET", "/api/nodes", nil)
+	r.Header.Set("Authorization", "Bearer "+token)
+	return r
+}
+
+func TestOIDCAuthenticate(t *testing.T) {
+	iss := newFakeIssuer(t)
+	o := NewOIDC(iss.srv.URL, "my-client")
+
+	valid := map[string]any{
+		"iss":   iss.srv.URL,
+		"sub":   "user-1",
+		"email": "user-1@example.com",
+		"name":  "User One",
+		"aud":   "my-client",
+		"exp":   time.Now().Add(time.Hour).Unix(),
+		"scope": "openid profile",
+	}
+
+	p, err := o.Authenticate(bearerReq(iss.mint(t, valid)))
+	if err != nil {
+		t.Fatalf("valid token rejected: %v", err)
+	}
+	if p.Subject != "user-1" || p.Kind != KindOIDC || !p.HasScope("profile") {
+		t.Fatalf("unexpected principal: %+v", p)
+	}
+
+	// VerifyClaims returns the rich claim set incl. email/name.
+	rawTok := iss.mint(t, valid)
+	c, err := o.VerifyClaims(rawTok)
+	if err != nil {
+		t.Fatalf("VerifyClaims rejected valid token: %v", err)
+	}
+	if c.Subject != "user-1" || c.Email != "user-1@example.com" || c.Name != "User One" || len(c.Scopes) != 2 {
+		t.Fatalf("unexpected claims: %+v", c)
+	}
+
+	// aud as array form also accepted
+	arr := map[string]any{
+		"iss": iss.srv.URL, "sub": "user-1", "aud": []string{"other", "my-client"},
+		"exp": time.Now().Add(time.Hour).Unix(),
+	}
+	if _, err := o.Authenticate(bearerReq(iss.mint(t, arr))); err != nil {
+		t.Fatalf("array aud rejected: %v", err)
+	}
+
+	cases := map[string]map[string]any{
+		"expired":        {"iss": iss.srv.URL, "sub": "u", "aud": "my-client", "exp": time.Now().Add(-time.Hour).Unix()},
+		"wrong issuer":   {"iss": "https://evil.example", "sub": "u", "aud": "my-client", "exp": time.Now().Add(time.Hour).Unix()},
+		"wrong audience": {"iss": iss.srv.URL, "sub": "u", "aud": "not-me", "exp": time.Now().Add(time.Hour).Unix()},
+	}
+	for name, c := range cases {
+		if _, err := o.Authenticate(bearerReq(iss.mint(t, c))); err == nil {
+			t.Errorf("%s token accepted", name)
+		}
+		if _, err := o.VerifyClaims(iss.mint(t, c)); err == nil {
+			t.Errorf("VerifyClaims accepted %s token", name)
+		}
+	}
+
+	// tampered payload must fail signature verification
+	tok := iss.mint(t, valid)
+	tampered := fmt.Sprintf("%s.%s.%s",
+		tok[:len(tok)/3],
+		base64.RawURLEncoding.EncodeToString([]byte(`{"iss":"x","sub":"hacker"}`)),
+		"AAAA")
+	if _, err := o.Authenticate(bearerReq(tampered)); err == nil {
+		t.Error("tampered token accepted")
+	}
+	if _, err := o.VerifyClaims(tampered); err == nil {
+		t.Error("VerifyClaims accepted tampered token")
+	}
+
+	// no credentials → ErrNoCredentials so Chain falls through
+	if _, err := o.Authenticate(httptest.NewRequest("GET", "/", nil)); err == nil {
+		t.Error("missing bearer accepted")
+	}
+	if _, err := o.VerifyClaims(""); err == nil {
+		t.Error("VerifyClaims accepted empty token")
+	}
+}
+
+// --- API keys ---
+
+func TestKeyStoreMintAuthenticateRevoke(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "apikeys.json")
+	ks, err := OpenKeyStore(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	key, rec, err := ks.Mint("ci", []string{"nodes:read"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	a := APIKeyAuth{Keys: ks}
+	r := httptest.NewRequest("GET", "/api/nodes", nil)
+	r.Header.Set("X-API-Key", key)
+	p, err := a.Authenticate(r)
+	if err != nil {
+		t.Fatalf("minted key rejected: %v", err)
+	}
+	if p.Subject != rec.ID || p.Kind != KindAPIKey || !p.HasScope("nodes:read") {
+		t.Fatalf("unexpected principal: %+v", p)
+	}
+
+	// persists across reopen
+	ks2, err := OpenKeyStore(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := (APIKeyAuth{Keys: ks2}).Authenticate(r); err != nil {
+		t.Fatalf("key lost after reopen: %v", err)
+	}
+
+	// invalid key rejected
+	bad := httptest.NewRequest("GET", "/", nil)
+	bad.Header.Set("X-API-Key", "gck_not_a_real_key")
+	if _, err := a.Authenticate(bad); err == nil {
+		t.Error("invalid key accepted")
+	}
+
+	// revoked key rejected
+	if err := ks.Revoke(rec.ID); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := a.Authenticate(r); err == nil {
+		t.Error("revoked key accepted")
+	}
+	if err := ks.Revoke(rec.ID); err == nil {
+		t.Error("revoking twice should fail")
+	}
+}
+
+func TestChainBearerThenAPIKey(t *testing.T) {
+	iss := newFakeIssuer(t)
+	ks, err := OpenKeyStore(filepath.Join(t.TempDir(), "k.json"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	key, _, err := ks.Mint("integration", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	chain := Chain{NewOIDC(iss.srv.URL, ""), APIKeyAuth{Keys: ks}}
+
+	r := httptest.NewRequest("GET", "/", nil)
+	r.Header.Set("X-API-Key", key)
+	p, err := chain.Authenticate(r)
+	if err != nil {
+		t.Fatalf("api key via chain rejected: %v", err)
+	}
+	if p.Kind != KindAPIKey {
+		t.Fatalf("expected apikey principal, got %+v", p)
+	}
+
+	tok := iss.mint(t, map[string]any{"iss": iss.srv.URL, "sub": "u1", "exp": time.Now().Add(time.Hour).Unix()})
+	p, err = chain.Authenticate(bearerReq(tok))
+	if err != nil {
+		t.Fatalf("bearer via chain rejected: %v", err)
+	}
+	if p.Kind != KindOIDC {
+		t.Fatalf("expected oidc principal, got %+v", p)
+	}
+
+	if _, err := chain.Authenticate(httptest.NewRequest("GET", "/", nil)); err == nil {
+		t.Error("credential-less request accepted by chain")
+	}
+}
+
+// --- middleware ---
+
+func TestBearerMiddleware(t *testing.T) {
+	iss := newFakeIssuer(t)
+	tok := iss.mint(t, map[string]any{
+		"iss": iss.srv.URL, "sub": "u", "aud": "client",
+		"exp": time.Now().Add(time.Hour).Unix(),
+	})
+	a := NewOIDC(iss.srv.URL, "client")
+	hits := 0
+	handler := BearerMiddleware(a, http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		hits++
+		w.WriteHeader(http.StatusOK)
+	}))
+
+	// valid token → passes through
+	w := httptest.NewRecorder()
+	handler.ServeHTTP(w, bearerReq(tok))
+	if w.Code != http.StatusOK || hits != 1 {
+		t.Fatalf("valid token: code=%d hits=%d", w.Code, hits)
+	}
+
+	// missing token → 401
+	w = httptest.NewRecorder()
+	handler.ServeHTTP(w, httptest.NewRequest("GET", "/", nil))
+	if w.Code != http.StatusUnauthorized || hits != 1 {
+		t.Fatalf("missing token: code=%d hits=%d", w.Code, hits)
+	}
+}
+
+func TestWriteMiddlewarePassesReads(t *testing.T) {
+	a := NewOIDC("https://issuer.example", "")
+	hits := 0
+	handler := WriteMiddleware(a, http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) { hits++ }))
+
+	for _, m := range []string{http.MethodGet, http.MethodHead, http.MethodOptions} {
+		w := httptest.NewRecorder()
+		handler.ServeHTTP(w, httptest.NewRequest(m, "/", nil))
+		if w.Code != http.StatusOK {
+			t.Errorf("%s: code=%d, want 200", m, w.Code)
+		}
+	}
+}
diff --git a/auth/middleware.go b/auth/middleware.go
new file mode 100644
index 0000000..0fa2f19
--- /dev/null
+++ b/auth/middleware.go
@@ -0,0 +1,35 @@
+package auth
+
+import "net/http"
+
+// BearerMiddleware wraps next, requiring a valid bearer credential from a.
+// On success the request is forwarded unchanged; on failure a 401 is
+// returned.
+func BearerMiddleware(a Authenticator, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if _, err := a.Authenticate(r); err != nil {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
+// WriteMiddleware wraps next, requiring a valid bearer credential from a
+// only for mutating HTTP methods (POST, PUT, PATCH, DELETE). Safe methods
+// (GET, HEAD, OPTIONS) pass through unauthenticated — suitable for
+// services that serve public reads but gate writes behind auth.
+func WriteMiddleware(a Authenticator, next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.Method {
+		case http.MethodGet, http.MethodHead, http.MethodOptions:
+			next.ServeHTTP(w, r)
+		default:
+			if _, err := a.Authenticate(r); err != nil {
+				http.Error(w, "unauthorized", http.StatusUnauthorized)
+				return
+			}
+			next.ServeHTTP(w, r)
+		}
+	})
+}
diff --git a/auth/oidc.go b/auth/oidc.go
new file mode 100644
index 0000000..251c610
--- /dev/null
+++ b/auth/oidc.go
@@ -0,0 +1,337 @@
+package auth
+
+import (
+	"crypto"
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rsa"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"math/big"
+	"net/http"
+	"slices"
+	"strings"
+	"sync"
+	"time"
+)
+
+// OIDC validates `Authorization: Bearer <jwt>` against any OIDC issuer: it
+// fetches `<issuer>/.well-known/openid-configuration` once, caches the JWKS,
+// and verifies signature (RS256/ES256), iss, aud and exp/nbf. Zitadel is the
+// reference issuer but nothing here is provider-specific.
+//
+// Note: the issuer must mint JWT access tokens (for Zitadel, set the
+// application's access-token type to JWT); opaque tokens require
+// introspection, which this deliberately does not implement.
+type OIDC struct {
+	issuer   string
+	audience string // optional: when set, must appear in the token's aud
+	client   *http.Client
+
+	mu          sync.Mutex
+	jwksURI     string
+	keys        map[string]crypto.PublicKey // kid → verification key
+	lastRefresh time.Time
+
+	now func() time.Time // injectable clock for tests
+}
+
+var _ Authenticator = (*OIDC)(nil)
+
+// NewOIDC builds an OIDC authenticator for the given issuer URL. audience
+// is the expected `aud` claim (typically the OAuth client id); empty skips
+// the audience check.
+func NewOIDC(issuer, audience string) *OIDC {
+	return &OIDC{
+		issuer:   strings.TrimSuffix(issuer, "/"),
+		audience: audience,
+		client:   &http.Client{Timeout: 10 * time.Second},
+		now:      time.Now,
+	}
+}
+
+// Authenticate validates the bearer JWT on r and returns the caller as a
+// minimal Principal. For callers that need the full parsed claim set
+// (Email/Name/scope list) — e.g. admin handlers rendering who is logged in —
+// see VerifyClaims.
+func (o *OIDC) Authenticate(r *http.Request) (*Principal, error) {
+	h := r.Header.Get("Authorization")
+	token, ok := strings.CutPrefix(h, "Bearer ")
+	if !ok || token == "" {
+		return nil, ErrNoCredentials
+	}
+	c, err := o.VerifyClaims(token)
+	if err != nil {
+		return nil, err
+	}
+	return &Principal{Subject: c.Subject, Scopes: c.Scopes, Kind: KindOIDC}, nil
+}
+
+// Claims is the parsed, verified claim set returned by VerifyClaims. Only
+// the subset services commonly need is exposed; the raw claim JSON is
+// available via the original token, which callers can re-decode if they
+// need something custom.
+type Claims struct {
+	Issuer    string   `json:"iss"`
+	Subject   string   `json:"sub"`
+	Email     string   `json:"email,omitempty"`
+	Name      string   `json:"name,omitempty"`
+	Audience  Audience `json:"aud"`
+	Expires   int64    `json:"exp"`
+	NotBefore int64    `json:"nbf"`
+	Scopes    []string `json:"scope,omitempty"` // parsed from "scope" space-separated claim
+}
+
+// Audience unmarshals the `aud` claim, which RFC 7519 allows to be either a
+// string or an array of strings (some providers emit one, some the other;
+// Zitadel emits an array).
+type Audience []string
+
+func (a *Audience) UnmarshalJSON(b []byte) error {
+	var s string
+	if err := json.Unmarshal(b, &s); err == nil {
+		*a = Audience{s}
+		return nil
+	}
+	var ss []string
+	if err := json.Unmarshal(b, &ss); err != nil {
+		return err
+	}
+	*a = Audience(ss)
+	return nil
+}
+
+// contains reports whether want appears in the audience list.
+func (a Audience) contains(want string) bool {
+	return slices.Contains(a, want)
+}
+
+// clockSkew is the tolerance applied when checking expiry/notBefore so a
+// small clock drift between issuer and verifier doesn't reject a fresh
+// token.
+const clockSkew = 30 * time.Second
+
+// VerifyClaims verifies a raw JWT (no "Bearer " prefix) end-to-end and
+// returns the parsed claim set. Callers that aren't using the standard
+// Authorization header (e.g. an admin UI reading a session cookie) can
+// pass the cookie value directly. The token is verified for issuer,
+// audience (if configured), expiry/notBefore, and signature via the JWKS
+// cached against the configured issuer.
+func (o *OIDC) VerifyClaims(token string) (*Claims, error) {
+	parts := strings.Split(token, ".")
+	if len(parts) != 3 {
+		return nil, errors.New("oidc: malformed jwt")
+	}
+	headerJSON, err := base64.RawURLEncoding.DecodeString(parts[0])
+	if err != nil {
+		return nil, fmt.Errorf("oidc: bad jwt header: %w", err)
+	}
+	var header struct {
+		Alg string `json:"alg"`
+		Kid string `json:"kid"`
+	}
+	if err := json.Unmarshal(headerJSON, &header); err != nil {
+		return nil, fmt.Errorf("oidc: bad jwt header: %w", err)
+	}
+	payloadJSON, err := base64.RawURLEncoding.DecodeString(parts[1])
+	if err != nil {
+		return nil, fmt.Errorf("oidc: bad jwt payload: %w", err)
+	}
+	sig, err := base64.RawURLEncoding.DecodeString(parts[2])
+	if err != nil {
+		return nil, fmt.Errorf("oidc: bad jwt signature: %w", err)
+	}
+
+	key, err := o.key(header.Kid)
+	if err != nil {
+		return nil, err
+	}
+	digest := sha256.Sum256([]byte(parts[0] + "." + parts[1]))
+	switch header.Alg {
+	case "RS256":
+		pub, ok := key.(*rsa.PublicKey)
+		if !ok {
+			return nil, errors.New("oidc: key/alg mismatch")
+		}
+		if err := rsa.VerifyPKCS1v15(pub, crypto.SHA256, digest[:], sig); err != nil {
+			return nil, errors.New("oidc: invalid signature")
+		}
+	case "ES256":
+		pub, ok := key.(*ecdsa.PublicKey)
+		if !ok {
+			return nil, errors.New("oidc: key/alg mismatch")
+		}
+		if len(sig) != 64 {
+			return nil, errors.New("oidc: invalid signature")
+		}
+		rr := new(big.Int).SetBytes(sig[:32])
+		ss := new(big.Int).SetBytes(sig[32:])
+		if !ecdsa.Verify(pub, digest[:], rr, ss) {
+			return nil, errors.New("oidc: invalid signature")
+		}
+	default:
+		return nil, fmt.Errorf("oidc: unsupported alg %q", header.Alg)
+	}
+
+	// Decode into a wide struct so Email/Name are picked up regardless of
+	// claim shape (some providers omit them, some populate them).
+	var raw struct {
+		Issuer    string   `json:"iss"`
+		Subject   string   `json:"sub"`
+		Email     string   `json:"email"`
+		Name      string   `json:"name"`
+		Audience  Audience `json:"aud"`
+		Expires   int64    `json:"exp"`
+		NotBefore int64    `json:"nbf"`
+		Scope     string   `json:"scope"`
+	}
+	if err := json.Unmarshal(payloadJSON, &raw); err != nil {
+		return nil, fmt.Errorf("oidc: bad claims: %w", err)
+	}
+	now := o.now()
+	if strings.TrimSuffix(raw.Issuer, "/") != o.issuer {
+		return nil, errors.New("oidc: wrong issuer")
+	}
+	if raw.Expires != 0 && now.After(time.Unix(raw.Expires, 0).Add(clockSkew)) {
+		return nil, errors.New("oidc: token expired")
+	}
+	if raw.NotBefore != 0 && now.Add(clockSkew).Before(time.Unix(raw.NotBefore, 0)) {
+		return nil, errors.New("oidc: token not yet valid")
+	}
+	if o.audience != "" && !raw.Audience.contains(o.audience) {
+		return nil, errors.New("oidc: wrong audience")
+	}
+
+	var scopes []string
+	if raw.Scope != "" {
+		scopes = strings.Fields(raw.Scope)
+	}
+	return &Claims{
+		Issuer:    raw.Issuer,
+		Subject:   raw.Subject,
+		Email:     raw.Email,
+		Name:      raw.Name,
+		Audience:  raw.Audience,
+		Expires:   raw.Expires,
+		NotBefore: raw.NotBefore,
+		Scopes:    scopes,
+	}, nil
+}
+
+// key returns the verification key for kid, fetching/refreshing the JWKS
+// when the kid is unknown (key rotation), rate-limited to once a minute.
+func (o *OIDC) key(kid string) (crypto.PublicKey, error) {
+	o.mu.Lock()
+	defer o.mu.Unlock()
+	if k, ok := o.keys[kid]; ok {
+		return k, nil
+	}
+	if time.Since(o.lastRefresh) < time.Minute {
+		return nil, fmt.Errorf("oidc: unknown key id %q", kid)
+	}
+	if err := o.refreshLocked(); err != nil {
+		return nil, err
+	}
+	if k, ok := o.keys[kid]; ok {
+		return k, nil
+	}
+	return nil, fmt.Errorf("oidc: unknown key id %q", kid)
+}
+
+// refreshLocked (re)fetches the discovery document (first time) and the
+// JWKS. Callers must hold o.mu.
+func (o *OIDC) refreshLocked() error {
+	o.lastRefresh = o.now()
+	if o.jwksURI == "" {
+		var disc struct {
+			JWKSURI string `json:"jwks_uri"`
+		}
+		if err := o.getJSON(o.issuer+"/.well-known/openid-configuration", &disc); err != nil {
+			return fmt.Errorf("oidc: discovery: %w", err)
+		}
+		if disc.JWKSURI == "" {
+			return errors.New("oidc: discovery document has no jwks_uri")
+		}
+		o.jwksURI = disc.JWKSURI
+	}
+	var set struct {
+		Keys []jwk `json:"keys"`
+	}
+	if err := o.getJSON(o.jwksURI, &set); err != nil {
+		return fmt.Errorf("oidc: jwks: %w", err)
+	}
+	keys := make(map[string]crypto.PublicKey, len(set.Keys))
+	for _, k := range set.Keys {
+		pub, err := k.publicKey()
+		if err != nil {
+			continue // skip unsupported key types (e.g. enc-use keys)
+		}
+		keys[k.Kid] = pub
+	}
+	o.keys = keys
+	return nil
+}
+
+func (o *OIDC) getJSON(url string, v any) error {
+	res, err := o.client.Get(url)
+	if err != nil {
+		return err
+	}
+	defer res.Body.Close()
+	if res.StatusCode != http.StatusOK {
+		return fmt.Errorf("GET %s: %s", url, res.Status)
+	}
+	return json.NewDecoder(res.Body).Decode(v)
+}
+
+// jwk is the subset of RFC 7517 needed to build RSA and P-256 keys.
+type jwk struct {
+	Kty string `json:"kty"`
+	Kid string `json:"kid"`
+	Use string `json:"use"`
+	N   string `json:"n"`
+	E   string `json:"e"`
+	Crv string `json:"crv"`
+	X   string `json:"x"`
+	Y   string `json:"y"`
+}
+
+func (k jwk) publicKey() (crypto.PublicKey, error) {
+	switch k.Kty {
+	case "RSA":
+		n, err := base64.RawURLEncoding.DecodeString(k.N)
+		if err != nil {
+			return nil, err
+		}
+		eBytes, err := base64.RawURLEncoding.DecodeString(k.E)
+		if err != nil {
+			return nil, err
+		}
+		return &rsa.PublicKey{
+			N: new(big.Int).SetBytes(n),
+			E: int(new(big.Int).SetBytes(eBytes).Int64()),
+		}, nil
+	case "EC":
+		if k.Crv != "P-256" {
+			return nil, fmt.Errorf("unsupported curve %q", k.Crv)
+		}
+		x, err := base64.RawURLEncoding.DecodeString(k.X)
+		if err != nil {
+			return nil, err
+		}
+		y, err := base64.RawURLEncoding.DecodeString(k.Y)
+		if err != nil {
+			return nil, err
+		}
+		return &ecdsa.PublicKey{
+			Curve: elliptic.P256(),
+			X:     new(big.Int).SetBytes(x),
+			Y:     new(big.Int).SetBytes(y),
+		}, nil
+	default:
+		return nil, fmt.Errorf("unsupported key type %q", k.Kty)
+	}
+}
diff --git a/catalog-emit b/catalog-emit
deleted file mode 100755
index 89dc757..0000000
Binary files a/catalog-emit and /dev/null differ
diff --git a/catalog/consumer.go b/catalog/consumer.go
index 54348e0..044a92a 100644
--- a/catalog/consumer.go
+++ b/catalog/consumer.go
@@ -1,330 +1,99 @@
 package catalog
 
-// ProjectionStore — the consumer half of the projection seam (see
-// docs/catalog-projection.md). A commerce service (cart, checkout, order, UCP)
-// feeds it the catalog.projection_published frames it receives off the bus and
-// reads catalog facts back by SKU. The store owns a LOCAL read-only projection:
+// Projection seam — catalog binding over the generic platform/projection
+// package. The value-agnostic machinery (snapshot .bin format, mmap reader,
+// race-free SnapshotHolder, the begin→chunk*→end consumer state machine with
+// epoch gating + delta overlay) lives in platform/projection. This file is the
+// thin catalog-specific binding: it fixes the WIRE type to catalog.Projection
+// and supplies the decoder for the catalog wire schema, so cart / checkout /
+// order / catalog-emit keep their existing catalog.* API unchanged.
 //
-//   - A full snapshot stream (begin → chunk* → end, one MetaEpoch) is assembled
-//     into the store's OWN .bin (SnapshotWriter), atomically swapped in as the
-//     mmap base (SnapshotHolder). The base is the cold-start substrate — it
-//     survives a restart (reopened from disk), so the consumer serves reads
-//     immediately without waiting for a bus replay.
-//   - Per-batch deltas of the SAME epoch are applied to an in-memory overlay on
-//     top of the base (overlay wins; a delete leaves a tombstone that shadows the
-//     base and the fallback). The next snapshot folds the overlay into a new base.
-//
-// Read order on Get: overlay → mmap base → optional caller fallback (e.g. an
-// HTTP fetch from the reader) — last-resort only. The access pattern is point
-// lookup by SKU; nothing enumerates the catalog.
-//
-// No shared volume: each consumer builds its own base from the bus. Concurrency:
-// HandleFrame is single-writer (drive it from one delivery goroutine); Get is
-// safe from many readers.
+// See docs/catalog-projection.md for the consumer contract and
+// docs/plan-projectionstore-generalization.md for why the split exists.
 
 import (
-	"fmt"
-	"log"
-	"os"
-	"path/filepath"
-	"strconv"
-	"strings"
-	"sync"
+	"time"
+
+	"git.k6n.net/mats/platform/projection"
 )
 
-// ProjectionFallback resolves an SKU the local store doesn't have yet (cold
-// start before the first snapshot, or an item added since the last base). It
-// returns a raw Projection — the store applies the SAME Transform to it, so a
-// fallback hit is shaped identically to bus-fed data. Return (_, false) to
-// report a genuine miss.
-type ProjectionFallback func(sku string) (Projection, bool)
+// Generic machinery, re-exported under their historical catalog names. These
+// are type aliases — catalog.SnapshotHolder[T] IS projection.SnapshotHolder[T],
+// so values flow between the two packages without conversion.
+type (
+	SnapshotWriter[T any] = projection.SnapshotWriter[T]
+	SnapshotReader[T any] = projection.SnapshotReader[T]
+	SnapshotHolder[T any] = projection.SnapshotHolder[T]
 
-// ProjectionTransform projects the wire Projection down to the consumer's own
-// stored type T. It is MANDATORY: it is the one place a consumer declares which
-// catalog facts it actually needs (and any derivation, e.g. tax_class → rate),
-// so its local .bin holds only that — smaller, and read-ready with no per-Get
-// conversion. Every value entering the store — snapshot, delta, and fallback —
-// passes through it exactly once.
-type ProjectionTransform[T any] func(Projection) T
+	// ProjectionStore is a consumer's local, cold-start-ready cache of T, fed the
+	// catalog.projection_published frames off the bus. The wire type is fixed to
+	// catalog.Projection; T is the consumer's projected shape.
+	ProjectionStore[T any] = projection.Store[Projection, T]
 
-// ProjectionStore is a consumer's local, cold-start-ready projection cache of T.
-type ProjectionStore[T any] struct {
-	dir, name string
-	transform ProjectionTransform[T]
-	fallback  ProjectionFallback
-	holder    *SnapshotHolder[T] // mmap base; atomic-swap + drain-grace
+	// ProjectionTransform projects the wire Projection down to the consumer's
+	// stored type T (the one place a consumer declares which catalog facts it
+	// needs). MANDATORY.
+	ProjectionTransform[T any] = projection.Transform[Projection, T]
 
-	mu      sync.RWMutex
-	epoch   int64                      // epoch of the live base
-	overlay map[string]overlayEntry[T] // same-epoch deltas layered on the base
+	// ProjectionFallback resolves a SKU the local store doesn't have yet. May be nil.
+	ProjectionFallback = projection.Fallback[Projection]
+)
 
-	// in-progress snapshot assembly (one epoch at a time)
-	building      *SnapshotWriter[T]
-	buildingEpoch int64
-	buildingSeen  int
+// DefaultDrainGrace is the SnapshotHolder default drain period (re-exported).
+const DefaultDrainGrace = projection.DefaultDrainGrace
+
+// Snapshot file errors, re-exported so callers can errors.Is against the
+// catalog.* names. Identity is preserved (these ARE the projection package's
+// error values).
+var (
+	ErrSnapshotMagic     = projection.ErrSnapshotMagic
+	ErrSnapshotVersion   = projection.ErrSnapshotVersion
+	ErrSnapshotTruncated = projection.ErrSnapshotTruncated
+	ErrSnapshotBadBlob   = projection.ErrSnapshotBadBlob
+	ErrSnapshotOverflow  = projection.ErrSnapshotOverflow
+	ErrSKUTooLong        = projection.ErrSKUTooLong
+	ErrTenantTooLong     = projection.ErrTenantTooLong
+)
+
+// NewSnapshotWriter returns an empty snapshot writer for value type T.
+func NewSnapshotWriter[T any]() *SnapshotWriter[T] { return projection.NewSnapshotWriter[T]() }
+
+// NewSnapshotReader wraps an already-loaded []byte as a reader of T.
+func NewSnapshotReader[T any](data []byte) (*SnapshotReader[T], error) {
+	return projection.NewSnapshotReader[T](data)
 }
 
-type overlayEntry[T any] struct {
-	val     T
-	deleted bool
+// ReadSnapshotFile opens + parses a snapshot file as a reader of T (mmap on Unix).
+func ReadSnapshotFile[T any](path string) (*SnapshotReader[T], error) {
+	return projection.ReadSnapshotFile[T](path)
 }
 
-// NewProjectionStore opens a store of T backed by dir/name (its own .bin).
-// transform is REQUIRED — it defines the consumer's projection shape (see
-// ProjectionTransform). If a base from a previous run is present it is reopened
-// immediately (cold-start ready); its epoch is read from the sidecar so
-// same-epoch deltas can resume. fallback may be nil. Close() when done.
+// NewSnapshotHolder returns a holder of T with the given drain grace.
+func NewSnapshotHolder[T any](grace time.Duration) *SnapshotHolder[T] {
+	return projection.NewSnapshotHolder[T](grace)
+}
+
+// NewProjectionStore opens a catalog ProjectionStore backed by dir/name (its own
+// .bin). transform is REQUIRED (declares the stored shape); fallback may be nil.
+// If a base from a previous run is present it is reopened immediately
+// (cold-start ready). Close() when done.
 func NewProjectionStore[T any](dir, name string, transform ProjectionTransform[T], fallback ProjectionFallback) (*ProjectionStore[T], error) {
-	if dir == "" || name == "" {
-		return nil, fmt.Errorf("catalog: ProjectionStore needs dir and name")
-	}
-	if transform == nil {
-		return nil, fmt.Errorf("catalog: ProjectionStore needs a transform (declare which catalog facts this consumer stores)")
-	}
-	s := &ProjectionStore[T]{
-		dir:       dir,
-		name:      name,
-		transform: transform,
-		fallback:  fallback,
-		holder:    NewSnapshotHolder[T](0),
-		overlay:   make(map[string]overlayEntry[T]),
-	}
-	path := filepath.Join(dir, name)
-	if err := s.holder.Open(path); err == nil {
-		s.epoch = readEpochSidecar(path)
-		log.Printf("catalog: ProjectionStore reopened base %s (epoch=%d, %d entries)", path, s.epoch, s.holder.Len())
-	} else if !os.IsNotExist(err) {
-		// A corrupt/partial base on disk shouldn't crash boot — start empty and
-		// wait for the next snapshot; reads fall back meanwhile.
-		log.Printf("catalog: ProjectionStore ignoring unreadable base %s: %v", path, err)
-	}
-	return s, nil
+	return projection.NewStore[Projection, T](dir, name, decodeProjectionUpdates, transform, fallback)
 }
 
-// HandleFrame applies one catalog.projection_published event (snapshot frame or
-// delta), routed by its Meta. Drive this from a single delivery goroutine. A
-// returned error means the payload was undecodable (the caller may NACK); a
-// frame that simply doesn't apply (stale epoch, wrong assembly state) returns
-// nil so the delivery is acked.
-func (s *ProjectionStore[T]) HandleFrame(meta map[string]string, payload []byte) error {
-	epoch, _ := strconv.ParseInt(meta[MetaEpoch], 10, 64)
-	switch meta[MetaSnapshotPhase] {
-	case SnapshotBegin:
-		return s.beginSnapshot(epoch, meta[MetaTenant])
-	case SnapshotChunk:
-		return s.chunkSnapshot(epoch, payload)
-	case SnapshotEnd:
-		return s.endSnapshot(epoch, meta)
-	case "":
-		return s.applyDelta(epoch, payload)
-	default:
-		return nil // unknown phase — ignore forward-compatibly
-	}
-}
-
-func (s *ProjectionStore[T]) beginSnapshot(epoch int64, tenant string) error {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	if epoch <= s.epoch {
-		return nil // we already hold this base or newer
-	}
-	// Start (or restart) assembly for this epoch. A new begin supersedes any
-	// half-built snapshot (e.g. a producer that aborted mid-stream).
-	s.building = NewSnapshotWriter[T]().WithTenant(tenant)
-	s.buildingEpoch = epoch
-	s.buildingSeen = 0
-	return nil
-}
-
-func (s *ProjectionStore[T]) chunkSnapshot(epoch int64, payload []byte) error {
-	ups, err := DecodeUpdates(payload)
+// decodeProjectionUpdates bridges the catalog wire schema ([]ProjectionUpdate —
+// a flat Projection plus a `deleted` flag) to the generic projection.Update the
+// Store works with. The index key is the Projection's SKU. This is the one
+// catalog-specific seam the generic Store can't know: the on-the-wire payload
+// shape is a domain decision.
+func decodeProjectionUpdates(payload []byte) ([]projection.Update[Projection], error) {
+	raw, err := DecodeUpdates(payload)
 	if err != nil {
-		return fmt.Errorf("catalog: decode snapshot chunk: %w", err)
+		return nil, err
 	}
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	if s.building == nil || epoch != s.buildingEpoch {
-		return nil // not assembling this epoch
+	out := make([]projection.Update[Projection], len(raw))
+	for i, u := range raw {
+		out[i] = projection.Update[Projection]{Key: u.SKU, Value: u.Projection, Deleted: u.Deleted}
 	}
-	for _, u := range ups {
-		if u.Deleted || u.SKU == "" {
-			continue // a snapshot carries live items only
-		}
-		// Transform applied here — the base stores the consumer's shape, not the
-		// full Projection.
-		if err := s.building.Append(u.SKU, s.transform(u.Projection)); err != nil {
-			log.Printf("catalog: ProjectionStore skip %q in snapshot: %v", u.SKU, err)
-			continue
-		}
-		s.buildingSeen++
-	}
-	return nil
-}
-
-func (s *ProjectionStore[T]) endSnapshot(epoch int64, meta map[string]string) error {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	if s.building == nil || epoch != s.buildingEpoch {
-		return nil
-	}
-	w := s.building
-	s.building = nil
-
-	// Completeness guard: if begin/end advertised a count and we appended fewer
-	// (a lost chunk), DON'T swap — keep the old base and wait for the next
-	// snapshot. Serving an incomplete catalog silently is worse than stale.
-	if cs := meta[MetaCount]; cs != "" {
-		if want, perr := strconv.Atoi(cs); perr == nil && want != s.buildingSeen {
-			log.Printf("catalog: ProjectionStore snapshot epoch=%d incomplete (%d/%d) — discarding, keeping base epoch=%d",
-				epoch, s.buildingSeen, want, s.epoch)
-			return nil
-		}
-	}
-
-	path, err := w.Write(s.dir, s.name)
-	if err != nil {
-		return fmt.Errorf("catalog: write snapshot base: %w", err)
-	}
-	// Persist the epoch sidecar (atomic) BEFORE swapping the in-memory reader, so
-	// a reopened store's epoch always matches the .bin already on disk (w.Write
-	// renamed the new file into place above). Writing it after Reload would leave
-	// a crash window where the new .bin carries a stale/absent epoch — the store
-	// would then mislabel its base and drop the current epoch's deltas on restart.
-	writeEpochSidecar(path, epoch)
-	if err := s.holder.Reload(path); err != nil {
-		return fmt.Errorf("catalog: reload snapshot base: %w", err)
-	}
-	s.epoch = epoch
-	s.overlay = make(map[string]overlayEntry[T]) // base now includes everything ≤ epoch
-	log.Printf("catalog: ProjectionStore snapshot applied — base swapped epoch=%d (%d entries)", epoch, s.buildingSeen)
-	return nil
-}
-
-func (s *ProjectionStore[T]) applyDelta(epoch int64, payload []byte) error {
-	ups, err := DecodeUpdates(payload)
-	if err != nil {
-		return fmt.Errorf("catalog: decode delta: %w", err)
-	}
-	s.mu.Lock()
-	defer s.mu.Unlock()
-	switch {
-	case s.building != nil:
-		// Under FIFO delivery a delta won't interleave a snapshot stream; if it
-		// does, the snapshot is authoritative for its epoch, so drop the delta.
-		return nil
-	case epoch < s.epoch:
-		return nil // stale — predates our base
-	case epoch > s.epoch:
-		// CONTRACT: a future-epoch delta is DROPPED, not buffered. Under FIFO
-		// per-queue delivery begin(N) always precedes delta(N), so this only
-		// happens after a reconnect that missed the whole snapshot — which a
-		// buffer couldn't reassemble anyway. Recovery is the next snapshot of
-		// this epoch; the consumer's fallback covers reads until then.
-		return nil
-	}
-	var upserts, deletes int
-	for _, u := range ups {
-		if u.SKU == "" {
-			continue
-		}
-		if u.Deleted {
-			s.overlay[u.SKU] = overlayEntry[T]{deleted: true}
-			deletes++
-			continue
-		}
-		// Transform applied here too — overlay holds the consumer's shape.
-		s.overlay[u.SKU] = overlayEntry[T]{val: s.transform(u.Projection)}
-		upserts++
-	}
-	if upserts+deletes > 0 {
-		log.Printf("catalog: ProjectionStore delta applied epoch=%d (+%d/-%d, overlay=%d)",
-			epoch, upserts, deletes, len(s.overlay))
-	}
-	return nil
-}
-
-// Get resolves a value by SKU: overlay (incl. tombstone) → mmap base →
-// fallback. A tombstoned SKU returns (zero, false) and never reaches the
-// fallback — the catalog explicitly removed it. A fallback hit is run through
-// the same Transform, so it's shaped like everything else.
-func (s *ProjectionStore[T]) Get(sku string) (T, bool) {
-	var zero T
-	s.mu.RLock()
-	e, ok := s.overlay[sku]
-	s.mu.RUnlock()
-	if ok {
-		if e.deleted {
-			return zero, false
-		}
-		return e.val, true
-	}
-	if v, ok := s.holder.Get(sku); ok {
-		return v, true
-	}
-	if s.fallback != nil {
-		if p, ok := s.fallback(sku); ok {
-			return s.transform(p), true
-		}
-	}
-	return zero, false
-}
-
-// IsDeleted reports whether the overlay holds a tombstone for sku — a delete
-// seen on the bus since the current base. Lets a caller skip a downstream fetch
-// for an SKU the catalog just removed. After a snapshot folds the delete into a
-// new base the SKU is simply absent (IsDeleted → false); "absent" is then the
-// caller's fetch/miss path, which a deleted item resolves as a downstream 404.
-func (s *ProjectionStore[T]) IsDeleted(sku string) bool {
-	s.mu.RLock()
-	defer s.mu.RUnlock()
-	e, ok := s.overlay[sku]
-	return ok && e.deleted
-}
-
-// Epoch returns the epoch of the current base (0 if none yet).
-func (s *ProjectionStore[T]) Epoch() int64 {
-	s.mu.RLock()
-	defer s.mu.RUnlock()
-	return s.epoch
-}
-
-// Stats returns observability counters (base size, overlay size, epoch).
-func (s *ProjectionStore[T]) Stats() (epoch int64, baseCount, overlayCount int) {
-	s.mu.RLock()
-	defer s.mu.RUnlock()
-	return s.epoch, s.holder.Len(), len(s.overlay)
-}
-
-// Close releases the mmap base.
-func (s *ProjectionStore[T]) Close() error { return s.holder.Close() }
-
-// ── epoch sidecar ────────────────────────────────────────────────────────────
-// The .bin header has no room for the epoch, so it's persisted next to it as
-// "<name>.epoch" — read on boot so a reopened base can resume same-epoch deltas.
-
-func epochSidecarPath(binPath string) string { return binPath + ".epoch" }
-
-// writeEpochSidecar writes the epoch atomically (temp+rename) so a crash mid-write
-// can't leave a torn file that parses as a bogus epoch — a reader either sees the
-// old epoch or the new one, never garbage.
-func writeEpochSidecar(binPath string, epoch int64) {
-	p := epochSidecarPath(binPath)
-	tmp := p + ".tmp"
-	if err := os.WriteFile(tmp, []byte(strconv.FormatInt(epoch, 10)), 0o644); err != nil {
-		log.Printf("catalog: ProjectionStore epoch sidecar write: %v", err)
-		return
-	}
-	if err := os.Rename(tmp, p); err != nil {
-		log.Printf("catalog: ProjectionStore epoch sidecar rename: %v", err)
-		_ = os.Remove(tmp)
-	}
-}
-
-func readEpochSidecar(binPath string) int64 {
-	b, err := os.ReadFile(epochSidecarPath(binPath))
-	if err != nil {
-		return 0
-	}
-	n, _ := strconv.ParseInt(strings.TrimSpace(string(b)), 10, 64)
-	return n
+	return out, nil
 }
diff --git a/catalog/projection.go b/catalog/projection.go
index 2d14794..ee22087 100644
--- a/catalog/projection.go
+++ b/catalog/projection.go
@@ -36,6 +36,7 @@ import (
 	"encoding/json"
 
 	"git.k6n.net/mats/platform/money"
+	"git.k6n.net/mats/platform/projection"
 )
 
 // Projection is the published, versioned catalog entry that commerce modules
@@ -82,10 +83,10 @@ import (
 // the JSON token. The on-disk snapshot binary header/record layout is
 // unchanged \u2014 only the per-blob JSON payload gains a key.
 type Projection struct {
-	ItemID uint32 `json:"item_id,omitempty"`
-	ID     string `json:"id"`
-	SKU    string `json:"sku"`
-	Title  string `json:"title,omitempty"`
+	ItemID      uint32      `json:"item_id,omitempty"`
+	ID          string      `json:"id"`
+	SKU         string      `json:"sku"`
+	Title       string      `json:"title,omitempty"`
 	PriceIncVat money.Cents `json:"price_inc_vat"`
 	Currency    string      `json:"currency,omitempty"`
 	// TaxClass uses a string identifier (see platform/tax class constants).
@@ -154,7 +155,9 @@ func Decode(raw []byte) (Projection, error) {
 // catalog.projection_published carries two channels distinguished only by
 // event.Event.Meta. The producer (slask-finder writeradmin) sets these keys and
 // the consumer (ProjectionStore, consumer.go) reads them — one definition so the
-// two never drift. See docs/catalog-projection.md.
+// two never drift. The framing itself is domain-neutral and lives in
+// platform/projection; catalog re-exports the constants so the producer/consumer
+// keep their catalog.* names. See docs/catalog-projection.md.
 //
 //   - A FULL SNAPSHOT is a framed begin → chunk* → end stream, all tagged with one
 //     MetaEpoch. begin/end carry MetaCount (total projections) for completeness
@@ -162,13 +165,13 @@ func Decode(raw []byte) (Projection, error) {
 //   - A DELTA has NO MetaSnapshotPhase key, and MetaEpoch names the snapshot base
 //     it builds on (consumers ignore a delta older than the base they hold).
 const (
-	MetaSnapshotPhase = "snapshot" // begin|chunk|end; absent ⇒ delta
-	MetaEpoch         = "epoch"     // int64 (Unix-ms), monotonic across rebuilds
-	MetaSeq           = "seq"       // chunk sequence within a snapshot
-	MetaCount         = "count"     // total projections (begin/end only)
-	MetaTenant        = "tenant"    // namespace tag
+	MetaSnapshotPhase = projection.MetaSnapshotPhase // begin|chunk|end; absent ⇒ delta
+	MetaEpoch         = projection.MetaEpoch         // int64 (Unix-ms), monotonic across rebuilds
+	MetaSeq           = projection.MetaSeq           // chunk sequence within a snapshot
+	MetaCount         = projection.MetaCount         // total projections (begin/end only)
+	MetaTenant        = projection.MetaTenant        // namespace tag
 
-	SnapshotBegin = "begin"
-	SnapshotChunk = "chunk"
-	SnapshotEnd   = "end"
+	SnapshotBegin = projection.SnapshotBegin
+	SnapshotChunk = projection.SnapshotChunk
+	SnapshotEnd   = projection.SnapshotEnd
 )
diff --git a/catalog/snapshot_helpers_test.go b/catalog/snapshot_helpers_test.go
deleted file mode 100644
index ad5337a..0000000
--- a/catalog/snapshot_helpers_test.go
+++ /dev/null
@@ -1,28 +0,0 @@
-// Local helpers for snapshot_test.go — production code does not see them.
-package catalog
-
-import (
-	"os"
-)
-
-func os2WriteFile(path string, data []byte, perm os.FileMode) error {
-	return os.WriteFile(path, data, perm)
-}
-
-func os2Stat(path string) (os.FileInfo, error) {
-	return os.Stat(path)
-}
-
-// appendP appends a Projection keyed by its own SKU — the producer/test pattern
-// over the now-generic SnapshotWriter[Projection] (Append takes an explicit key).
-func appendP(w *SnapshotWriter[Projection], p Projection) error {
-	return w.Append(p.SKU, p)
-}
-
-func repeat32(s string, n int) string {
-	out := make([]byte, 0, n*len(s))
-	for i := 0; i < n; i++ {
-		out = append(out, s...)
-	}
-	return string(out)
-}
diff --git a/cmd/catalog-emit/main.go b/cmd/catalog-emit/main.go
index 0a4f768..bdf4bec 100644
--- a/cmd/catalog-emit/main.go
+++ b/cmd/catalog-emit/main.go
@@ -82,7 +82,7 @@ type TenantState struct {
 	mu    sync.RWMutex
 	items map[string]catalog.Projection
 
-	holder *catalog.SnapshotHolder
+	holder *catalog.SnapshotHolder[catalog.Projection]
 	path   string // <out-dir>/<tenant>.bin
 
 	rebuildCount atomic.Uint64
@@ -92,7 +92,7 @@ type TenantState struct {
 
 // newTenantState wires a brand-new tenant. Caller is responsible for
 // calling rebuild() (or Open via the initial state pipeline) before any Get.
-func newTenantState(tenant string, sch catalog.Schema, holder *catalog.SnapshotHolder, path string) *TenantState {
+func newTenantState(tenant string, sch catalog.Schema, holder *catalog.SnapshotHolder[catalog.Projection], path string) *TenantState {
 	return &TenantState{
 		tenant: tenant,
 		schema: sch,
@@ -148,9 +148,9 @@ func (ts *TenantState) rebuild(batch []catalog.ProjectionUpdate) error {
 // writeSnapshot emits snapshot to disk (atomic temp+rename) and reloads
 // the holder. Called WITHOUT holding ts.mu.
 func (ts *TenantState) writeSnapshot(snap []catalog.Projection) error {
-	w := catalog.NewSnapshotWriter().WithTenant(ts.tenant)
+	w := catalog.NewSnapshotWriter[catalog.Projection]().WithTenant(ts.tenant)
 	for _, p := range snap {
-		if err := w.Append(p); err != nil {
+		if err := w.Append(p.SKU, p); err != nil {
 			return fmt.Errorf("append %s: %w", p.SKU, err)
 		}
 	}
@@ -260,7 +260,7 @@ func main() {
 			sch.Tenant = t
 		}
 		path := filepath.Join(*outDir, t+".bin")
-		holder := catalog.NewSnapshotHolder(*drainGrace)
+		holder := catalog.NewSnapshotHolder[catalog.Projection](*drainGrace)
 		states[t] = newTenantState(t, sch, holder, path)
 	}
 	// Register cleanup for both the defer path (graceful shutdown / SIGINT)
diff --git a/go.mod b/go.mod
index ee6c8b9..47b85e9 100644
--- a/go.mod
+++ b/go.mod
@@ -3,6 +3,6 @@ module git.k6n.net/mats/platform
 go 1.26.2
 
 require (
-	github.com/rabbitmq/amqp091-go v1.11.0
+	github.com/rabbitmq/amqp091-go v1.12.0
 	golang.org/x/sys v0.46.0
 )
diff --git a/go.sum b/go.sum
index ffa9cde..c26aeed 100644
--- a/go.sum
+++ b/go.sum
@@ -1,5 +1,4 @@
-github.com/rabbitmq/amqp091-go v1.11.0 h1:HxIctVm9Gid/Vtn706necmZ7Wj6pgGI2eqplRbEY8O8=
-github.com/rabbitmq/amqp091-go v1.11.0/go.mod h1:Hy4jKW5kQART1u+JkDTF9YYOQUHXqMuhrgxOEeS7G4o=
+github.com/rabbitmq/amqp091-go v1.12.0 h1:V0v14Iqfs+MwHWihJt/nGS5Ulu0vw572b2Co3mwunkI=
 go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
 go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
 golang.org/x/sys v0.46.0 h1:noSf2Fq6F8DBgS+LysIkx7rIExoNHJsxOAtPp4rthXw=
diff --git a/inventory/reservation.go b/inventory/reservation.go
new file mode 100644
index 0000000..20e2087
--- /dev/null
+++ b/inventory/reservation.go
@@ -0,0 +1,59 @@
+package inventory
+
+import (
+	"context"
+	"time"
+)
+
+// Value types for inventory reservation — cross-module vocabulary shared by
+// cart, checkout, backoffice, and the inventory service. Moving them here
+// means consumers depend on platform/inventory rather than on the Redis
+// implementation package (go-redis-inventory).
+
+// SKU identifies a single stock-keeping unit.
+type SKU string
+
+// LocationID identifies a geographic stock location (e.g. "se", "no").
+type LocationID string
+
+// CartID identifies a shopping cart — used as the reservation holder key.
+type CartID string
+
+// InventoryReference identifies a specific SKU at a specific location.
+type InventoryReference struct {
+	SKU        SKU
+	LocationID LocationID
+}
+
+// CartReserveRequest is what a caller provides to reserve stock for a cart.
+type CartReserveRequest struct {
+	*InventoryReference
+	CartID   CartID
+	Quantity uint32
+	TTL      time.Duration
+}
+
+// ReservationPolicy is the cross-module seam for inventory reservation.
+// Cart mutations call Reserve on add-to-cart / change-quantity and Release on
+// item removal. Checkout calls Available to validate stock before a payment
+// session, and Release on cancel / destroy. Every implementation behind this
+// interface is free to choose its internal storage (Redis Lua, in-memory map,
+// actor grain, etc.) — the consumer never needs to know.
+//
+// This is the "policy-first seam" from docs/ARCHITECTURE.md:
+// commerce policy lives here; the fast, contended storage lives behind it.
+type ReservationPolicy interface {
+	// Reserve holds qty units of sku at locationID for the caller's cart.
+	// Returns an error if insufficient stock is available.
+	Reserve(ctx context.Context, req CartReserveRequest) error
+
+	// Release frees a prior reservation for the given cart at the given
+	// sku+location. Idempotent: releasing an already-released or expired
+	// reservation is a no-op.
+	Release(ctx context.Context, sku SKU, locationID LocationID, cartID CartID) error
+
+	// Available returns the quantity that can still be reserved right now —
+	// total stock minus all active reservations. Used by checkout to validate
+	// that the cart's items are still reservable before opening a payment session.
+	Available(ctx context.Context, sku SKU, locationID LocationID) (int64, error)
+}
diff --git a/inventory/reservation_test.go b/inventory/reservation_test.go
new file mode 100644
index 0000000..9b50eaf
--- /dev/null
+++ b/inventory/reservation_test.go
@@ -0,0 +1,187 @@
+package inventory
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// InMemoryReservationPolicy is a thread-safe in-memory implementation of
+// ReservationPolicy for tests — no Redis needed. Backed by a map from
+// SKU+location to available quantity.
+type InMemoryReservationPolicy struct {
+	mu   sync.Mutex
+	qty  map[string]int64 // key: "sku|location" → available quantity
+	held map[string]int64 // key: "sku|location|cartID" → reserved quantity
+}
+
+// NewInMemoryReservationPolicy creates a policy seeded with the given
+// sku+location → available quantity entries. Each entry is a pair of
+// (sku, location, qty); pass in threes.
+func NewInMemoryReservationPolicy(seed ...any) *InMemoryReservationPolicy {
+	p := &InMemoryReservationPolicy{
+		qty:  make(map[string]int64),
+		held: make(map[string]int64),
+	}
+	for i := 0; i+2 < len(seed); i += 3 {
+		sku, ok := seed[i].(SKU)
+		if !ok {
+			continue
+		}
+		loc, ok := seed[i+1].(LocationID)
+		if !ok {
+			continue
+		}
+		qty, ok := seed[i+2].(int64)
+		if !ok {
+			continue
+		}
+		p.qty[key(sku, loc)] = qty
+	}
+	return p
+}
+
+func key(sku SKU, loc LocationID) string    { return string(sku) + "|" + string(loc) }
+func heldKey(sku SKU, loc LocationID, c CartID) string {
+	return string(sku) + "|" + string(loc) + "|" + string(c)
+}
+
+func (p *InMemoryReservationPolicy) Reserve(_ context.Context, req CartReserveRequest) error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	k := key(req.SKU, req.LocationID)
+	avail := p.qty[k]
+	hk := heldKey(req.SKU, req.LocationID, req.CartID)
+	if _, exists := p.held[hk]; exists {
+		// Already held — add to existing reservation
+		avail -= p.held[hk]
+	}
+	if avail < int64(req.Quantity) {
+		return ErrInsufficientStock
+	}
+	p.held[hk] += int64(req.Quantity)
+	return nil
+}
+
+func (p *InMemoryReservationPolicy) Release(_ context.Context, sku SKU, locationID LocationID, cartID CartID) error {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	delete(p.held, heldKey(sku, locationID, cartID))
+	return nil
+}
+
+func (p *InMemoryReservationPolicy) Available(_ context.Context, sku SKU, locationID LocationID) (int64, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	k := key(sku, locationID)
+	avail := p.qty[k]
+	// Subtract all held quantities for this sku+location
+	for hk, held := range p.held {
+		if len(hk) > len(k) && hk[:len(k)] == k && hk[len(k)] == '|' {
+			avail -= held
+		}
+	}
+	if avail < 0 {
+		avail = 0
+	}
+	return avail, nil
+}
+
+// Reset clears all reservations and resets available quantities.
+func (p *InMemoryReservationPolicy) Reset() {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.held = make(map[string]int64)
+}
+
+// ErrInsufficientStock is returned by Reserve when not enough stock is available.
+var ErrInsufficientStock = &reservationError{"insufficient stock"}
+
+type reservationError struct{ msg string }
+
+func (e *reservationError) Error() string { return e.msg }
+
+// ── Tests ────────────────────────────────────────────────────────────────
+
+func TestInMemoryReserveAndRelease(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryReservationPolicy(
+		SKU("ABC-1"), LocationID("se"), int64(10),
+		SKU("ABC-1"), LocationID("no"), int64(5),
+	)
+
+	// Reserve available stock
+	err := p.Reserve(ctx, CartReserveRequest{
+		InventoryReference: &InventoryReference{SKU: "ABC-1", LocationID: "se"},
+		CartID:             "cart-1",
+		Quantity:           3,
+		TTL:                15 * time.Minute,
+	})
+	if err != nil {
+		t.Fatalf("Reserve: %v", err)
+	}
+	avail, _ := p.Available(ctx, "ABC-1", "se")
+	if avail != 7 {
+		t.Fatalf("after reserve 3 of 10, expected 7 available, got %d", avail)
+	}
+
+	// Release
+	if err := p.Release(ctx, "ABC-1", "se", "cart-1"); err != nil {
+		t.Fatalf("Release: %v", err)
+	}
+	avail, _ = p.Available(ctx, "ABC-1", "se")
+	if avail != 10 {
+		t.Fatalf("after release, expected 10 available, got %d", avail)
+	}
+}
+
+func TestInMemoryInsufficientStock(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryReservationPolicy(
+		SKU("ABC-1"), LocationID("se"), int64(2),
+	)
+
+	err := p.Reserve(ctx, CartReserveRequest{
+		InventoryReference: &InventoryReference{SKU: "ABC-1", LocationID: "se"},
+		CartID:             "cart-1",
+		Quantity:           5,
+		TTL:                15 * time.Minute,
+	})
+	if err == nil {
+		t.Fatal("expected insufficient stock error, got nil")
+	}
+}
+
+func TestInMemoryConcurrentReserves(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryReservationPolicy(
+		SKU("ABC-1"), LocationID("se"), int64(100),
+	)
+
+	errs := make(chan error, 10)
+	for i := 0; i < 10; i++ {
+		go func(n int) {
+			errs <- p.Reserve(ctx, CartReserveRequest{
+				InventoryReference: &InventoryReference{SKU: "ABC-1", LocationID: "se"},
+				CartID:             CartID(string(rune('0' + n))),
+				Quantity:           10,
+				TTL:                15 * time.Minute,
+			})
+		}(i)
+	}
+
+	for i := 0; i < 10; i++ {
+		if err := <-errs; err != nil {
+			t.Errorf("concurrent reserve %d: %v", i, err)
+		}
+	}
+
+	avail, _ := p.Available(ctx, "ABC-1", "se")
+	if avail != 0 {
+		t.Fatalf("after 10×10 reserves from 100, expected 0 available, got %d", avail)
+	}
+}
diff --git a/order/order.go b/order/order.go
index 3f445fe..d687ce8 100644
--- a/order/order.go
+++ b/order/order.go
@@ -16,14 +16,18 @@ package order
 import "encoding/json"
 
 // Line is one ordered line item, reduced to what reactors need: the SKU, how
-// many units sold, and the inventory location to commit against. Pricing/tax
-// live in the order grain, not on this seam.
+// many units sold, the inventory location to commit against, and whether the
+// item is drop-ship (fulfilled by the supplier, never decremented from local
+// stock). Pricing/tax live in the order grain, not on this seam.
 type Line struct {
 	SKU      string `json:"sku"`
 	Quantity int    `json:"quantity"`
 	// Location is the inventory location / store id to decrement. Empty means
 	// "use the order's Country" (central stock) — see Created.Country.
 	Location string `json:"location,omitempty"`
+	// DropShip marks items fulfilled directly by the supplier. The inventory
+	// reactor must skip the stock decrement for these lines.
+	DropShip bool `json:"drop_ship,omitempty"`
 }
 
 // Created is the payload of an event.OrderCreated envelope.
diff --git a/projection/doc.go b/projection/doc.go
new file mode 100644
index 0000000..e6fa8db
--- /dev/null
+++ b/projection/doc.go
@@ -0,0 +1,27 @@
+// Package projection is the domain-neutral substrate for the "snapshot stream +
+// local index" seam: one authoritative producer publishes a framed
+// begin → chunk* → end snapshot (plus per-batch deltas) on the bus, and N
+// receivers each assemble their OWN cold-start-ready, mmap-backed .bin index
+// from it. No shared volume; each consumer is independently eventually
+// consistent.
+//
+// It was extracted from platform/catalog (which now re-exports these types for
+// its Projection wire shape) so other domains can ride the same rails without
+// depending on catalog. The pieces:
+//
+//   - SnapshotWriter[T] / SnapshotReader[T] (snapshot.go) — the value-agnostic
+//     key→blob .bin format: sorted records, binary-search Get, full-walk Scan,
+//     mmap on Unix. Generic over the stored type T.
+//   - SnapshotHolder[T] (manager.go) — race-free atomic hot-reload of a reader
+//     under concurrent Get/Scan, with a drain-grace close-batching reaper.
+//   - Store[W, T] (store.go) — the consumer state machine: snapshot assembly,
+//     epoch gating, same-epoch delta overlay (with delete tombstones), and the
+//     completeness guard. Generic over the WIRE type W (decoded by a
+//     domain-supplied Decoder) and the STORED type T (projected by a Transform).
+//   - The bus framing contract (framing.go) — Meta* keys, snapshot phases, and
+//     the decoded Update[W] shape.
+//
+// The framing is fixed and shared; the on-the-wire payload schema is the
+// domain's own concern, bridged by Decoder[W]. See
+// docs/plan-projectionstore-generalization.md for why this boundary exists.
+package projection
diff --git a/projection/framing.go b/projection/framing.go
new file mode 100644
index 0000000..c5d11f4
--- /dev/null
+++ b/projection/framing.go
@@ -0,0 +1,57 @@
+package projection
+
+// Bus framing contract (shared by producer + consumer).
+//
+// A projection stream carries two channels distinguished only by the event's
+// Meta map. The producer sets these keys and the consumer reads them — one
+// definition so the two never drift.
+//
+//   - A FULL SNAPSHOT is a framed begin → chunk* → end stream, all tagged with one
+//     MetaEpoch. begin/end carry MetaCount (total entries) for completeness
+//     validation; chunks carry the entries.
+//   - A DELTA has NO MetaSnapshotPhase key, and MetaEpoch names the snapshot base
+//     it builds on (consumers ignore a delta older than the base they hold).
+//
+// The framing is domain-neutral. What rides inside each chunk/delta payload is
+// a domain-specific wire schema, decoded by the Decoder the consumer supplies
+// (see Store / Decoder). catalog re-exports these constants for wire-compat.
+const (
+	MetaSnapshotPhase = "snapshot" // begin|chunk|end; absent ⇒ delta
+	MetaEpoch         = "epoch"    // int64 (Unix-ms), monotonic across rebuilds
+	MetaSeq           = "seq"      // chunk sequence within a snapshot
+	MetaCount         = "count"    // total entries (begin/end only)
+	MetaTenant        = "tenant"   // namespace tag
+
+	SnapshotBegin = "begin"
+	SnapshotChunk = "chunk"
+	SnapshotEnd   = "end"
+)
+
+// Update is the decoded form of one entry in a snapshot chunk or a delta. Key
+// is the index key; Value is the domain wire value; Deleted marks a removal
+// (the consumer tombstones the key in its overlay, or skips it in a snapshot).
+// This is the in-memory shape the framing layer works with — the on-the-wire
+// JSON schema is the domain's own concern, bridged by a Decoder.
+type Update[W any] struct {
+	Key     string
+	Value   W
+	Deleted bool
+}
+
+// Decoder turns one chunk/delta payload (the event's Payload bytes) into
+// decoded Updates. Each domain defines its own wire schema and supplies the
+// decoder; the Store owns only the framing/assembly around it. A decode error
+// is surfaced from HandleFrame so the caller may NACK.
+type Decoder[W any] func(payload []byte) ([]Update[W], error)
+
+// Transform projects a wire value W down to the consumer's stored type T. It is
+// the one place a consumer declares which facts it actually needs (and any
+// derivation), so its local .bin holds only that. Every value entering the
+// store — snapshot, delta, and fallback — passes through it exactly once.
+type Transform[W, T any] func(W) T
+
+// Fallback resolves a key the local store doesn't have yet (cold start before
+// the first snapshot, or a key added since the last base). It returns a raw
+// wire value W — the store applies the same Transform to it. Return (_, false)
+// for a genuine miss. May be nil.
+type Fallback[W any] func(key string) (W, bool)
diff --git a/projection/helpers_test.go b/projection/helpers_test.go
new file mode 100644
index 0000000..167c34c
--- /dev/null
+++ b/projection/helpers_test.go
@@ -0,0 +1,54 @@
+// Local helpers for the snapshot / holder tests — production code does not see
+// them. tProj is a stand-in stored value; the projection package is
+// value-agnostic, so the tests use a small local struct instead of depending on
+// catalog.Projection (the type these tests were extracted alongside).
+package projection
+
+import "os"
+
+// tProj mirrors the fields the ported catalog snapshot tests exercise. Prices
+// are plain int64 here (no money dependency in the generic package).
+type tProj struct {
+	ID          string `json:"id"`
+	SKU         string `json:"sku"`
+	Title       string `json:"title,omitempty"`
+	PriceIncVat int64  `json:"price_inc_vat"`
+	Currency    string `json:"currency,omitempty"`
+	TaxClass    string `json:"tax_class,omitempty"`
+	UpdatedAt   int64  `json:"updated_at,omitempty"`
+}
+
+func os2WriteFile(path string, data []byte, perm os.FileMode) error {
+	return os.WriteFile(path, data, perm)
+}
+
+func os2Stat(path string) (os.FileInfo, error) {
+	return os.Stat(path)
+}
+
+// appendP appends a tProj keyed by its own SKU — the producer/test pattern over
+// the generic SnapshotWriter (Append takes an explicit key).
+func appendP(w *SnapshotWriter[tProj], p tProj) error {
+	return w.Append(p.SKU, p)
+}
+
+// makeProjection builds a tProj fixture.
+func makeProjection(sku string, price int64) tProj {
+	return tProj{
+		ID:          "sku:" + sku,
+		SKU:         sku,
+		Title:       "Item " + sku,
+		PriceIncVat: price,
+		Currency:    "SEK",
+		TaxClass:    "standard",
+		UpdatedAt:   1,
+	}
+}
+
+func repeat32(s string, n int) string {
+	out := make([]byte, 0, n*len(s))
+	for i := 0; i < n; i++ {
+		out = append(out, s...)
+	}
+	return string(out)
+}
diff --git a/catalog/snapshot_manager.go b/projection/manager.go
similarity index 89%
rename from catalog/snapshot_manager.go
rename to projection/manager.go
index 50104a3..068262b 100644
--- a/catalog/snapshot_manager.go
+++ b/projection/manager.go
@@ -12,21 +12,20 @@
 // The drain grace is retained as a close-BATCHING optimization: it defers and
 // coalesces reader closes onto the reaper tick instead of taking the exclusive
 // Lock on every swap. Pattern source: slask-finder's mmap+reload stores.
-// See docs/inventory-shape.md for the prior art.
 //
 // Use:
 //
-//	holder := catalog.NewSnapshotHolder(30 * time.Second) // drain grace
+//	holder := projection.NewSnapshotHolder[T](30 * time.Second) // drain grace
 //	if err := holder.Open("/var/lib/catalog/bygglagret.bin"); err != nil { ... }
 //	// later, when a fresh snapshot lands:
 //	if err := holder.Reload("/var/lib/catalog/bygglagret.bin"); err != nil { ... }
-//	// pulls the projection; always hits the current reader:
-//	p, ok := holder.Get(sku)
+//	// pulls the value; always hits the current reader:
+//	p, ok := holder.Get(key)
 //
-// holder.Get / Len are safe for parallel use. Open and Reload are also
+// holder.Get / Scan / Len are safe for parallel use. Open and Reload are also
 // safe for parallel use. Close stops the reaper goroutine and shuts the
 // current reader + drains down.
-package catalog
+package projection
 
 import (
 	"sync"
@@ -48,10 +47,7 @@ type SnapshotHolder[T any] struct {
 	// readMu makes reclamation race-free: Get holds RLock across the
 	// Load-and-read of a reader, and a reader is only Close()d (munmap) under
 	// Lock — so a reader can never be unmapped while a Get is reading it, even
-	// if that Get's goroutine was descheduled past the drain grace. The grace
-	// (below) is now only close-BATCHING — defer + coalesce closes off the swap
-	// path — NOT the safety mechanism it was before (which was best-effort and
-	// could SIGSEGV under a long enough scheduler pause).
+	// if that Get's goroutine was descheduled past the drain grace.
 	readMu sync.RWMutex
 
 	drainsMu   sync.Mutex
@@ -138,6 +134,20 @@ func (h *SnapshotHolder[T]) Get(sku string) (T, bool) {
 	return r.Get(sku)
 }
 
+// Scan walks every (key, value) of the current reader in key-sorted order.
+// Return false from fn to stop early. The RLock is held for the whole scan so
+// the reader can't be munmapped mid-walk — keep fn cheap (collect then sort
+// outside the callback for ranked output). No-op when the holder is empty.
+func (h *SnapshotHolder[T]) Scan(fn func(key string, v T) bool) {
+	h.readMu.RLock()
+	defer h.readMu.RUnlock()
+	r := h.current.Load()
+	if r == nil {
+		return
+	}
+	r.Scan(fn)
+}
+
 // Len returns the count of the current reader (0 if empty).
 func (h *SnapshotHolder[T]) Len() int {
 	h.readMu.RLock()
@@ -208,8 +218,6 @@ func (h *SnapshotHolder[T]) reapNow() {
 	h.drainsMu.Unlock()
 	for _, d := range reaped {
 		// Lock briefly so the munmap can't race an in-flight Get on this reader.
-		// Gets that loaded this reader hold RLock and finish before the Lock is
-		// granted; Gets starting after see the swapped-in reader, never this one.
 		h.readMu.Lock()
 		_ = d.r.Close()
 		h.readMu.Unlock()
diff --git a/catalog/snapshot_manager_test.go b/projection/manager_test.go
similarity index 78%
rename from catalog/snapshot_manager_test.go
rename to projection/manager_test.go
index 4048848..dbe8b27 100644
--- a/catalog/snapshot_manager_test.go
+++ b/projection/manager_test.go
@@ -1,4 +1,4 @@
-package catalog
+package projection
 
 import (
 	"fmt"
@@ -7,8 +7,6 @@ import (
 	"sync/atomic"
 	"testing"
 	"time"
-
-	"git.k6n.net/mats/platform/money"
 )
 
 // TestSnapshotHolder_SwapRace storms the holder: 100 readers Get in tight loops
@@ -17,13 +15,9 @@ import (
 //
 //   - No panic / SIGBUS — completion is the proof the drain grace never unmaps a
 //     reader while an in-flight Get is reading it.
-//   - No torn reads — every hit decodes to a valid Projection (SKU matches,
-//     price > 0). A read of a munmapped page would crash or fail to decode.
-//   - No gap — since every snapshot version contains every SKU, a Get must never
-//     miss; misses would mean the holder briefly exposed an empty/nil reader
-//     mid-swap. (Exact "stale beyond drain grace" timing is intentionally not
-//     asserted — it's timing-flaky, and the no-torn-read + no-crash checks
-//     already cover the drain-safety property it stands in for.)
+//   - No torn reads — every hit decodes to a valid value (SKU matches, price > 0).
+//   - No gap — since every snapshot version contains every key, a Get must never
+//     miss; misses would mean the holder briefly exposed an empty/nil reader.
 func TestSnapshotHolder_SwapRace(t *testing.T) {
 	dir := t.TempDir()
 
@@ -31,11 +25,11 @@ func TestSnapshotHolder_SwapRace(t *testing.T) {
 	const skuCount = 200
 	paths := make([]string, versions)
 	for v := 0; v < versions; v++ {
-		w := NewSnapshotWriter[Projection]()
+		w := NewSnapshotWriter[tProj]()
 		for i := 0; i < skuCount; i++ {
 			sku := fmt.Sprintf("SKU-%03d", i)
 			// price encodes the version (always > 0) so a torn read is detectable.
-			if err := w.Append(sku, Projection{ID: "sku:" + sku, SKU: sku, PriceIncVat: money.Cents(int64((v + 1) * 100))}); err != nil {
+			if err := w.Append(sku, tProj{ID: "sku:" + sku, SKU: sku, PriceIncVat: int64((v + 1) * 100)}); err != nil {
 				t.Fatal(err)
 			}
 		}
@@ -46,7 +40,7 @@ func TestSnapshotHolder_SwapRace(t *testing.T) {
 		paths[v] = p
 	}
 
-	h := NewSnapshotHolder[Projection](100 * time.Millisecond)
+	h := NewSnapshotHolder[tProj](100 * time.Millisecond)
 	if err := h.Open(paths[0]); err != nil {
 		t.Fatal(err)
 	}
@@ -111,7 +105,7 @@ func TestSnapshotHolder_SwapRace(t *testing.T) {
 		t.Fatal("torn/garbage read or reload error during swap storm")
 	}
 	if m := atomic.LoadInt64(&misses); m != 0 {
-		t.Fatalf("got %d misses — holder exposed an empty reader mid-swap (every version has every SKU)", m)
+		t.Fatalf("got %d misses — holder exposed an empty reader mid-swap (every version has every key)", m)
 	}
 	if atomic.LoadInt64(&reads) == 0 {
 		t.Fatal("no successful reads — test did not exercise the holder")
@@ -125,12 +119,10 @@ type fakeCloser struct{ hits int }
 
 func (f *fakeCloser) Close() error { f.hits++; return nil }
 
-// withFakeCloser swaps SnapshotReader's closer for our counter. Returns the
-// reader and the counter; tests then call holder.Set(reloaded) and assert
-// that close is invoked after the drain grace.
-func withFakeCloser(t *testing.T, path string, fc *fakeCloser) *SnapshotReader[Projection] {
+// withFakeCloser swaps SnapshotReader's closer for our counter.
+func withFakeCloser(t *testing.T, path string, fc *fakeCloser) *SnapshotReader[tProj] {
 	t.Helper()
-	r, err := ReadSnapshotFile[Projection](path)
+	r, err := ReadSnapshotFile[tProj](path)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -142,13 +134,11 @@ func withFakeCloser(t *testing.T, path string, fc *fakeCloser) *SnapshotReader[P
 // TestSnapshotHolderReloadDrainsOld — Set swaps the current reader; the
 // previous reader is queued. Before grace: still Get-able. After grace:
 // reaped and Close fires.
-//
-// We use a very short grace so the test doesn't sleep long.
 func TestSnapshotHolderReloadDrainsOld(t *testing.T) {
 	dir := t.TempDir()
 
-	// 1) Build a valid first snapshot (skus A-1 / A-2).
-	w := NewSnapshotWriter[Projection]()
+	// 1) Build a valid first snapshot (keys A-1 / A-2).
+	w := NewSnapshotWriter[tProj]()
 	if err := appendP(w, makeProjection("A-1", 100)); err != nil {
 		t.Fatal(err)
 	}
@@ -162,8 +152,8 @@ func TestSnapshotHolderReloadDrainsOld(t *testing.T) {
 	first := withFakeCloser(t, filepath.Join(dir, "first.bin"), firstCloser)
 	defer first.Close()
 
-	// 2) Build a second snapshot (skus B-1).
-	w2 := NewSnapshotWriter[Projection]()
+	// 2) Build a second snapshot (key B-1).
+	w2 := NewSnapshotWriter[tProj]()
 	if err := appendP(w2, makeProjection("B-1", 300)); err != nil {
 		t.Fatal(err)
 	}
@@ -174,7 +164,7 @@ func TestSnapshotHolderReloadDrainsOld(t *testing.T) {
 	second := withFakeCloser(t, filepath.Join(dir, "second.bin"), secondCloser)
 	defer second.Close()
 
-	h := NewSnapshotHolder[Projection](50 * time.Millisecond)
+	h := NewSnapshotHolder[tProj](50 * time.Millisecond)
 	defer h.Close()
 	h.Set(first)
 	if h.Len() != 2 {
@@ -209,7 +199,7 @@ func TestSnapshotHolderReloadDrainsOld(t *testing.T) {
 
 // TestSnapshotHolderEmptyGet — holder with no current serves empty.
 func TestSnapshotHolderEmptyGet(t *testing.T) {
-	h := NewSnapshotHolder[Projection](100 * time.Millisecond)
+	h := NewSnapshotHolder[tProj](100 * time.Millisecond)
 	defer h.Close()
 	if _, ok := h.Get("anything"); ok {
 		t.Error("empty holder should return false")
@@ -220,12 +210,11 @@ func TestSnapshotHolderEmptyGet(t *testing.T) {
 }
 
 // TestSnapshotHolderCloseImmediatelyReaps — Close with pending drains
-// should close them now (no grace wait). Confirms the drain queue is
-// drained on Close.
+// should close them now (no grace wait).
 func TestSnapshotHolderCloseImmediatelyReaps(t *testing.T) {
 	dir := t.TempDir()
 
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	if err := appendP(w, makeProjection("X-1", 1)); err != nil {
 		t.Fatal(err)
 	}
@@ -235,7 +224,7 @@ func TestSnapshotHolderCloseImmediatelyReaps(t *testing.T) {
 	fc1 := &fakeCloser{}
 	r1 := withFakeCloser(t, filepath.Join(dir, "x.bin"), fc1)
 
-	w2 := NewSnapshotWriter[Projection]()
+	w2 := NewSnapshotWriter[tProj]()
 	if err := appendP(w2, makeProjection("Y-1", 2)); err != nil {
 		t.Fatal(err)
 	}
@@ -245,7 +234,7 @@ func TestSnapshotHolderCloseImmediatelyReaps(t *testing.T) {
 	fc2 := &fakeCloser{}
 	r2 := withFakeCloser(t, filepath.Join(dir, "y.bin"), fc2)
 
-	h := NewSnapshotHolder[Projection](10 * time.Second) // long grace so reap wouldn't happen on its own
+	h := NewSnapshotHolder[tProj](10 * time.Second) // long grace so reap wouldn't happen on its own
 	h.Set(r1)
 	h.Set(r2)
 
@@ -261,11 +250,10 @@ func TestSnapshotHolderCloseImmediatelyReaps(t *testing.T) {
 }
 
 // TestSnapshotHolderIdentityReload — Open the same path twice; current reader
-// is replaced; DrainCount returns 1 (old queued). Old's data still served
-// until grace elapses.
+// is replaced; DrainCount returns 1 (old queued).
 func TestSnapshotHolderIdentityReload(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	if err := appendP(w, makeProjection("ID-1", 1)); err != nil {
 		t.Fatal(err)
 	}
@@ -273,7 +261,7 @@ func TestSnapshotHolderIdentityReload(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	h := NewSnapshotHolder[Projection](10 * time.Second)
+	h := NewSnapshotHolder[tProj](10 * time.Second)
 	defer h.Close()
 	if err := h.Reload(filepath.Join(dir, "id.bin")); err != nil {
 		t.Fatal(err)
diff --git a/catalog/snapshot.go b/projection/snapshot.go
similarity index 72%
rename from catalog/snapshot.go
rename to projection/snapshot.go
index 285047c..f7daba4 100644
--- a/catalog/snapshot.go
+++ b/projection/snapshot.go
@@ -1,41 +1,43 @@
-// Snapshot reader/writer for the published catalog Projection.
+// Snapshot reader/writer — a value-agnostic, mmap-friendly key→blob store.
 //
-// Why a snapshot file at all? The bus is the live channel, but it doesn't
-// carry the historical population on cold start. A snapshot file gives
-// consumers (cart / checkout / order / UCP add-to-cart) a fast mmap-loaded
-// cold-start substrate, so the first add-line request doesn't have to wait for
-// the bus to replay. The bus then keeps it fresh.
+// This is the domain-neutral substrate extracted from platform/catalog: the
+// bus is the live channel, but it doesn't carry the historical population on
+// cold start. A snapshot file gives a consumer a fast mmap-loaded cold-start
+// substrate, so the first read doesn't have to wait for the bus to replay. The
+// bus then keeps it fresh. catalog re-exports these types for its Projection;
+// other domains (tracking popularity/suggestions) instantiate them over their
+// own value type.
 //
 // Format v2 (little-endian, fixed-width, mmap-friendly):
 //
 //	header   [72 bytes]   magic(8) version(4) reserved(4) count(8)
 //	                       blobStart(8) recordStart(8) tenant(32, zero-padded)
-//	blobs    variable     count sequential JSON-encoded Projection bodies, no padding
-//	records  count*44     each: sku(32, zero-padded) blobOffset(8) blobLength(4);
-//	                       sorted by SKU via the single compareSKU total order
+//	blobs    variable     count sequential JSON-encoded value bodies, no padding
+//	records  count*44     each: key(32, zero-padded) blobOffset(8) blobLength(4);
+//	                       sorted by key via the single compareSKU total order
 //
 // Tenant field at offsets 40–71 (zero-padded) names the namespace this
-// snapshot covers. Independent SKU schemas can produce Projections into
-// different tenant files without conflict. Hot-reload across tenants stays
-// safe under SnapshotHolder (snapshot_manager.go).
+// snapshot covers. Independent key schemas can produce values into different
+// tenant files without conflict. Hot-reload across tenants stays safe under
+// SnapshotHolder (manager.go).
 //
-// Sorting + binary search gives O(log N) per-SKU lookup with no on-disk hash
-// table and no collisions. SKUs longer than 32 bytes are rejected at write
-// time (Cart SKUs in this codebase are well under that).
+// Sorting + binary search gives O(log N) per-key lookup with no on-disk hash
+// table and no collisions. Keys longer than 32 bytes are rejected at write
+// time. (Domains with longer natural keys — page paths, query strings — hash
+// the key to a fixed-width digest and carry the original inside the value blob.)
 //
 // Memory model: the Reader holds one []byte for the entire file (mmap on
-// Unix, ReadFile elsewhere). Per-SKU Get reads at most log2(N) record slots
-// (each 44 B) and one JSON blob (~200 B). For 220 k projections the
-// resident set is "pages touched this minute", not the whole file.
+// Unix, ReadFile elsewhere). Per-key Get reads at most log2(N) record slots
+// (each 44 B) and one JSON blob. Scan walks the sorted record section once.
 //
 // SAFETY: SnapshotReader alone is NOT safe across hot-reload. During reload,
-// in-flight Get() calls on the OLD reader continue to work until disposal
+// in-flight Get()/Scan() calls on the OLD reader continue to work until disposal
 // of the old reader is finalised; calling SnapshotReader.Close() concurrently
 // with Get() on the same reader risks unmapping pages-under-read (SIGBUS).
-// Use SnapshotHolder (snapshot_manager.go) for any reload-bearing setup —
-// it gives you an atomic.Pointer swap with a drain-grace (default 30s) so
-// concurrent Gets on the OLD reader finish naturally before munmap fires.
-package catalog
+// Use SnapshotHolder (manager.go) for any reload-bearing setup — it gives you
+// an atomic.Pointer swap with a drain-grace (default 30s) so concurrent Gets on
+// the OLD reader finish naturally before munmap fires.
+package projection
 
 import (
 	"encoding/binary"
@@ -52,31 +54,28 @@ import (
 const (
 	snapshotMagic   uint64 = 0x3230564a53435050 // "PCSJ002" v2 (was 0x3130564a53435050 = v1)
 	snapshotVersion uint32 = 2
-	maxSKULen             = 32
-	maxTenantLen          = 32
-	headerSize            = 8 + 4 + 4 + 8 + 8 + 8 + 32 // 72 bytes
-	recordSize            = 32 + 8 + 4                 // 44 bytes per record
+	maxSKULen              = 32
+	maxTenantLen           = 32
+	headerSize             = 8 + 4 + 4 + 8 + 8 + 8 + 32 // 72 bytes
+	recordSize             = 32 + 8 + 4                 // 44 bytes per record
 )
 
 var (
-	ErrSnapshotMagic     = errors.New("catalog: not a snapshot file (bad magic)")
-	ErrSnapshotVersion   = errors.New("catalog: unsupported snapshot version")
-	ErrSnapshotTruncated = errors.New("catalog: snapshot truncated")
-	ErrSnapshotBadBlob   = errors.New("catalog: snapshot record points past file")
-	ErrSnapshotOverflow  = errors.New("catalog: snapshot record-section size overflows uint64")
-	ErrSKUTooLong        = fmt.Errorf("catalog: SKU longer than %d bytes", maxSKULen)
-	ErrTenantTooLong     = fmt.Errorf("catalog: tenant name longer than %d bytes", maxTenantLen)
+	ErrSnapshotMagic     = errors.New("projection: not a snapshot file (bad magic)")
+	ErrSnapshotVersion   = errors.New("projection: unsupported snapshot version")
+	ErrSnapshotTruncated = errors.New("projection: snapshot truncated")
+	ErrSnapshotBadBlob   = errors.New("projection: snapshot record points past file")
+	ErrSnapshotOverflow  = errors.New("projection: snapshot record-section size overflows uint64")
+	ErrSKUTooLong        = fmt.Errorf("projection: key longer than %d bytes", maxSKULen)
+	ErrTenantTooLong     = fmt.Errorf("projection: tenant name longer than %d bytes", maxTenantLen)
 )
 
 // The snapshot is generic over the stored value T (see SnapshotWriter[T] /
 // SnapshotReader[T] below). The binary format, sorting, mmap, and bounds-check
 // logic is value-agnostic and lives in the unexported raw* layer; the generic
-// wrappers only marshal T → blob (write) and blob → T (read). The producer
-// stores catalog.Projection; each consumer stores its OWN projected type via a
-// transform (see ProjectionStore[T] in consumer.go) — that's the per-consumer
-// read-only projection from docs/catalog-projection.md.
+// wrappers only marshal T → blob (write) and blob → T (read).
 
-// rawSnapshotWriter is the value-agnostic writer: it indexes (sku → blob bytes).
+// rawSnapshotWriter is the value-agnostic writer: it indexes (key → blob bytes).
 // Single-shot: append() all, then write() once (atomic rename).
 type rawSnapshotWriter struct {
 	tenant  string
@@ -85,7 +84,7 @@ type rawSnapshotWriter struct {
 	dirty   bool
 }
 
-// snapshotRecord is the on-disk layout: SKU zero-padded to 32 bytes, then
+// snapshotRecord is the on-disk layout: key zero-padded to 32 bytes, then
 // an 8-byte offset (from file start) plus a 4-byte length for the JSON blob.
 type snapshotRecord struct {
 	SKU        [32]byte
@@ -93,7 +92,7 @@ type snapshotRecord struct {
 	BlobLength uint32
 }
 
-// append adds a pre-encoded blob under sku. ErrSKUTooLong if sku exceeds the
+// append adds a pre-encoded blob under key. ErrSKUTooLong if key exceeds the
 // on-disk width.
 func (w *rawSnapshotWriter) append(sku string, blob []byte) error {
 	if len(sku) > maxSKULen {
@@ -112,8 +111,7 @@ func (w *rawSnapshotWriter) append(sku string, blob []byte) error {
 
 // SnapshotWriter[T] is single-shot: Append() all values, then Write() once
 // (atomic rename-into-place so readers never see a partial file). T is the
-// stored value type (Projection for the producer; a consumer-specific type for
-// a ProjectionStore[T]).
+// stored value type.
 type SnapshotWriter[T any] struct {
 	raw rawSnapshotWriter
 }
@@ -132,16 +130,16 @@ func (w *SnapshotWriter[T]) WithTenant(tenant string) *SnapshotWriter[T] {
 	return w
 }
 
-// Append encodes v and indexes it under sku. Returns ErrSKUTooLong if sku
+// Append encodes v and indexes it under key. Returns ErrSKUTooLong if key
 // exceeds the on-disk width; returns marshal errors as wrapped fmt.Errorf.
-// SKU is explicit because T is opaque — the caller knows the key.
+// key is explicit because T is opaque — the caller knows the key.
 func (w *SnapshotWriter[T]) Append(sku string, v T) error {
 	if len(sku) > maxSKULen {
 		return ErrSKUTooLong
 	}
 	jb, err := json.Marshal(v)
 	if err != nil {
-		return fmt.Errorf("catalog: marshal snapshot value %q: %w", sku, err)
+		return fmt.Errorf("projection: marshal snapshot value %q: %w", sku, err)
 	}
 	return w.raw.append(sku, jb)
 }
@@ -155,7 +153,7 @@ func (w *SnapshotWriter[T]) Write(dir, name string) (finalPath string, err error
 }
 
 // write serialises to a temp file in dir, fsync's, then renames into the
-// final path. Records are sorted by SKU asc before serialisation so the
+// final path. Records are sorted by key asc before serialisation so the
 // reader's binary search works.
 func (w *rawSnapshotWriter) write(dir, name string) (finalPath string, err error) {
 	if w.dirty {
@@ -233,12 +231,11 @@ func (w *rawSnapshotWriter) write(dir, name string) (finalPath string, err error
 	return finalPath, nil
 }
 
-// compareSKU defines the SINGLE TOTAL ORDER for SKUs across both Writer
+// compareSKU defines the SINGLE TOTAL ORDER for keys across both Writer
 // (uses it for sort.Slice) and Reader (uses it for binary search). One
 // definition; invariant test verifies the on-disk result is sorted under
-// it. Source SKUs SHOULD NOT contain embedded null bytes — the padded
-// comparator treats them as terminators, which silently truncates an SKU.
-// Realistically no SKU has NUL.
+// it. Source keys SHOULD NOT contain embedded null bytes — the padded
+// comparator treats them as terminators, which silently truncates a key.
 func compareSKU(padded, other []byte) int {
 	// padded is a fixed-size zero-padded slot; trim trailing zeros.
 	end := len(padded)
@@ -272,16 +269,12 @@ func compareSKU(padded, other []byte) int {
 
 // compareRecordLookup is compareSKU specialised for Reader lookups so the
 // argument order matches the lex order: "padded" (fixed-slot) and "want"
-// (query).
-//
-// compareSKU is symmetric so this is just an alias used by Reader code
-// paths to make intent obvious. Any change to the total order here MUST be
-// reflected in Writer.sort.Slice and verified by the invariant test.
+// (query). compareSKU is symmetric so this is just an alias.
 func compareRecordLookup(padded, want []byte) int {
 	return compareSKU(padded, want)
 }
 
-// rawSnapshotReader is the value-agnostic reader: binary search by SKU, returns
+// rawSnapshotReader is the value-agnostic reader: binary search by key, returns
 // the raw blob bytes. Concurrency-safe for parallel getRaw; NOT safe racing
 // Close with getRaw (see package doc — use SnapshotHolder for reload).
 type rawSnapshotReader struct {
@@ -322,7 +315,7 @@ func NewSnapshotReader[T any](data []byte) (*SnapshotReader[T], error) {
 	return &SnapshotReader[T]{raw: rr}, nil
 }
 
-// Get returns the value for sku, or (zero, false) if absent / undecodable.
+// Get returns the value for key, or (zero, false) if absent / undecodable.
 func (r *SnapshotReader[T]) Get(sku string) (T, bool) {
 	var zero T
 	blob, ok := r.raw.getRaw(sku)
@@ -336,6 +329,22 @@ func (r *SnapshotReader[T]) Get(sku string) (T, bool) {
 	return v, true
 }
 
+// Scan walks every (key, value) in key-sorted order, calling fn for each.
+// Return false from fn to stop early. A value that fails to decode is skipped
+// (fn is not called for it). This is the enumeration / ranked-scan substrate:
+// callers that need top-N-by-value collect into a slice and sort in memory —
+// the on-disk order is by key, not value. NOT safe racing Close; drive it
+// through SnapshotHolder.Scan under reload.
+func (r *SnapshotReader[T]) Scan(fn func(key string, v T) bool) {
+	r.raw.scan(func(key string, blob []byte) bool {
+		var v T
+		if err := json.Unmarshal(blob, &v); err != nil {
+			return true // skip undecodable, keep going
+		}
+		return fn(key, v)
+	})
+}
+
 // Len returns the number of values in this snapshot.
 func (r *SnapshotReader[T]) Len() int { return r.raw.Len() }
 
@@ -379,7 +388,7 @@ func newRawSnapshotReader(data []byte) (*rawSnapshotReader, error) {
 	blobStart := binary.LittleEndian.Uint64(data[24:32])
 	recordStart := binary.LittleEndian.Uint64(data[32:40])
 	if blobStart != uint64(headerSize) {
-		return nil, fmt.Errorf("catalog: blobStart=%d expected %d", blobStart, headerSize)
+		return nil, fmt.Errorf("projection: blobStart=%d expected %d", blobStart, headerSize)
 	}
 
 	// Multiplication overflow guard: for a malformed/tampered count, the
@@ -402,10 +411,6 @@ func newRawSnapshotReader(data []byte) (*rawSnapshotReader, error) {
 	// Per-record blob-offset bounds check: defends against malformed /
 	// tampered files where the record section is intact but the blob
 	// pointer lies past EOF. A linear pass; cheap (44 B × count at open).
-	//
-	// Avoid the naive blobOff+blobLen > dataLen check — blobOff can be
-	// MaxUint64 in tampered input and uint64 addition wraps, falsely
-	// passing the test. Split the check into overflow-safe parts.
 	dataLen := uint64(len(data))
 	for i := uint64(0); i < count; i++ {
 		recOff := recordStart + i*uint64(recordSize)
@@ -439,9 +444,6 @@ func (r *rawSnapshotReader) Len() int { return int(r.count) }
 
 // Close releases the file handle and mmap region (if any). Idempotent — safe
 // to call from multiple goroutines (uses sync.Once under the hood).
-//
-// SAFETY: this is NOT safe against concurrent getRaw() — see package doc. Use
-// SnapshotHolder for any reload-bearing workflow.
 func (r *rawSnapshotReader) Close() error {
 	var err error
 	r.closeOnce.Do(func() {
@@ -452,7 +454,7 @@ func (r *rawSnapshotReader) Close() error {
 	return err
 }
 
-// getRaw returns the blob bytes for sku, or (nil, false) if not found. The
+// getRaw returns the blob bytes for key, or (nil, false) if not found. The
 // returned slice aliases the mmap region — valid until Close. O(log N) binary
 // search on the sorted record section.
 func (r *rawSnapshotReader) getRaw(sku string) ([]byte, bool) {
@@ -485,4 +487,21 @@ func (r *rawSnapshotReader) getRaw(sku string) ([]byte, bool) {
 	return nil, false
 }
 
-
+// scan walks every record in key-sorted order, yielding (key, blob). Return
+// false from fn to stop. The blob aliases the mmap region — valid until Close.
+func (r *rawSnapshotReader) scan(fn func(key string, blob []byte) bool) {
+	for i := uint64(0); i < r.count; i++ {
+		recOff := int(r.recordStart) + int(i)*recordSize
+		recSKU := r.data[recOff : recOff+32]
+		end := len(recSKU)
+		for end > 0 && recSKU[end-1] == 0 {
+			end--
+		}
+		key := string(recSKU[:end])
+		blobLen := binary.LittleEndian.Uint32(r.data[recOff+40 : recOff+44])
+		blobOff := binary.LittleEndian.Uint64(r.data[recOff+32 : recOff+40])
+		if !fn(key, r.data[blobOff:blobOff+uint64(blobLen)]) {
+			return
+		}
+	}
+}
diff --git a/catalog/snapshot_other.go b/projection/snapshot_other.go
similarity index 96%
rename from catalog/snapshot_other.go
rename to projection/snapshot_other.go
index 996abf9..9183d30 100644
--- a/catalog/snapshot_other.go
+++ b/projection/snapshot_other.go
@@ -4,7 +4,7 @@
 // identical to the unix mmap path — same reader code, same lookups — just
 // pays the full resident-set cost at Open. Production mmap-on-Linux/Darwin;
 // this exists for windows / plan9 / wasm builds.
-package catalog
+package projection
 
 import "os"
 
diff --git a/catalog/snapshot_test.go b/projection/snapshot_test.go
similarity index 76%
rename from catalog/snapshot_test.go
rename to projection/snapshot_test.go
index 78ea39b..cccf41b 100644
--- a/catalog/snapshot_test.go
+++ b/projection/snapshot_test.go
@@ -1,4 +1,4 @@
-package catalog
+package projection
 
 import (
 	"encoding/binary"
@@ -9,18 +9,16 @@ import (
 	"path/filepath"
 	"testing"
 	"time"
-
-	"git.k6n.net/mats/platform/money"
 )
 
-// TestSnapshotRoundTrip — write 1000 projections in shuffled order, read back
-// and verify all 1000 are found at the right SKU.
+// TestSnapshotRoundTrip — write 1000 values in shuffled order, read back
+// and verify all 1000 are found at the right key.
 func TestSnapshotRoundTrip(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "snap.bin")
 
 	const n = 1000
-	w := NewSnapshotWriter[Projection]().WithTenant("default")
+	w := NewSnapshotWriter[tProj]().WithTenant("default")
 	// intentionally shuffled input — sort is what reorders at Write.
 	perm := rand.Perm(n)
 	for _, i := range perm {
@@ -40,7 +38,7 @@ func TestSnapshotRoundTrip(t *testing.T) {
 		t.Errorf("Write returned %q expected %q", final, path)
 	}
 
-	r, err := ReadSnapshotFile[Projection](path)
+	r, err := ReadSnapshotFile[tProj](path)
 	if err != nil {
 		t.Fatalf("ReadSnapshotFile: %v", err)
 	}
@@ -53,7 +51,7 @@ func TestSnapshotRoundTrip(t *testing.T) {
 		t.Errorf("Tenant: got %q want %q", r.Tenant(), "default")
 	}
 
-	// every SKU present and correct
+	// every key present and correct
 	for _, i := range rand.Perm(n)[:50] { // probe 50 random lookups
 		sku := fmt.Sprintf("SKU-%05d", i)
 		got, ok := r.Get(sku)
@@ -61,25 +59,72 @@ func TestSnapshotRoundTrip(t *testing.T) {
 			t.Errorf("Get(%q) not found", sku)
 			continue
 		}
-		want := money.Cents(100 + (i%97)*100)
+		want := int64(100 + (i%97)*100)
 		if got.PriceIncVat != want {
 			t.Errorf("Get(%q) PriceIncVat: got %d want %d", sku, got.PriceIncVat, want)
 		}
 	}
 
-	// missing SKU
+	// missing key
 	if _, ok := r.Get("NOPE-99999"); ok {
-		t.Error("Get on missing SKU should return ok=false")
+		t.Error("Get on missing key should return ok=false")
+	}
+}
+
+// TestSnapshotScanWalksAllInKeyOrder — Scan visits every record exactly once in
+// ascending key order, and early-return stops the walk.
+func TestSnapshotScanWalksAllInKeyOrder(t *testing.T) {
+	dir := t.TempDir()
+	const n = 300
+	w := NewSnapshotWriter[tProj]()
+	for _, i := range rand.Perm(n) {
+		sku := fmt.Sprintf("K-%04d", i)
+		if err := appendP(w, makeProjection(sku, int64(i))); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if _, err := w.Write(dir, "scan.bin"); err != nil {
+		t.Fatal(err)
+	}
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "scan.bin"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer r.Close()
+
+	var keys []string
+	r.Scan(func(key string, v tProj) bool {
+		if v.SKU != key {
+			t.Errorf("Scan value/key mismatch: key=%q value.SKU=%q", key, v.SKU)
+		}
+		keys = append(keys, key)
+		return true
+	})
+	if len(keys) != n {
+		t.Fatalf("Scan visited %d records, want %d", len(keys), n)
+	}
+	for i := 1; i < len(keys); i++ {
+		if keys[i-1] >= keys[i] {
+			t.Fatalf("Scan not ascending at %d: %q >= %q", i, keys[i-1], keys[i])
+		}
+	}
+
+	// early-return stops the walk
+	count := 0
+	r.Scan(func(string, tProj) bool {
+		count++
+		return count < 5
+	})
+	if count != 5 {
+		t.Errorf("Scan early-return visited %d, want 5", count)
 	}
 }
 
 // TestSnapshotCompareSKUInvariant — verify the on-disk record section is
 // sorted under the same compareSKU total order that BinarySearch uses.
-// If sort.Slice and Reader.Get ever drift apart, lookups silently miss
-// — this test pins that invariant.
 func TestSnapshotCompareSKUInvariant(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	// deliberately inverted order after Append (so sort has work to do)
 	perm := rand.Perm(500)
 	for _, i := range perm {
@@ -91,13 +136,13 @@ func TestSnapshotCompareSKUInvariant(t *testing.T) {
 	if _, err := w.Write(dir, "snap.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r, err := ReadSnapshotFile[Projection](filepath.Join(dir, "snap.bin"))
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "snap.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer r.Close()
 
-	// Walk every record slot, harvest SKU bytes from the pad-then-trim
+	// Walk every record slot, harvest key bytes from the pad-then-trim
 	// form, and assert non-decreasing order under compareSKU.
 	prevSKU := ""
 	for i := 0; i < r.Len(); i++ {
@@ -127,18 +172,17 @@ func runesToPadded32(s string) []byte {
 }
 
 // TestSnapshotTenantRoundTrip — Writer.WithTenant lands in the header and
-// Reader surfaces it. Without this, one file per tenant is just a convention
-// with no on-disk evidence.
+// Reader surfaces it.
 func TestSnapshotTenantRoundTrip(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]().WithTenant("bygglagret")
+	w := NewSnapshotWriter[tProj]().WithTenant("bygglagret")
 	if err := appendP(w, makeProjection("ABC-1", 100)); err != nil {
 		t.Fatal(err)
 	}
 	if _, err := w.Write(dir, "snap.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r, err := ReadSnapshotFile[Projection](filepath.Join(dir, "snap.bin"))
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "snap.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -148,14 +192,14 @@ func TestSnapshotTenantRoundTrip(t *testing.T) {
 	}
 
 	// Empty tenant writes zero-padded.
-	w2 := NewSnapshotWriter[Projection]()
+	w2 := NewSnapshotWriter[tProj]()
 	if err := appendP(w2, makeProjection("ABC-2", 200)); err != nil {
 		t.Fatal(err)
 	}
 	if _, err := w2.Write(dir, "snap2.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r2, err := ReadSnapshotFile[Projection](filepath.Join(dir, "snap2.bin"))
+	r2, err := ReadSnapshotFile[tProj](filepath.Join(dir, "snap2.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -168,7 +212,7 @@ func TestSnapshotTenantRoundTrip(t *testing.T) {
 // TestSnapshotTenantTooLong — over-long tenant name rejected at Write.
 func TestSnapshotTenantTooLong(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]().WithTenant(string(make([]byte, 33)))
+	w := NewSnapshotWriter[tProj]().WithTenant(string(make([]byte, 33)))
 	if err := appendP(w, makeProjection("ABC", 1)); err != nil {
 		t.Fatal(err)
 	}
@@ -181,7 +225,7 @@ func TestSnapshotTenantTooLong(t *testing.T) {
 
 // TestSnapshotCountOverflowRejected — tampered count that would cause
 // recordStart + records*recordSize to overflow uint64 must be rejected
-// with ErrSnapshotOverflow (formerly a silent wraparound vulnerability).
+// with ErrSnapshotOverflow.
 func TestSnapshotCountOverflowRejected(t *testing.T) {
 	dir := t.TempDir()
 	path := filepath.Join(dir, "over.bin")
@@ -196,7 +240,7 @@ func TestSnapshotCountOverflowRejected(t *testing.T) {
 	if err := os2WriteFile(path, bs, 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotOverflow) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotOverflow) {
 		t.Errorf("overflow count rejected with %v, want ErrSnapshotOverflow", err)
 	}
 }
@@ -205,8 +249,8 @@ func TestSnapshotCountOverflowRejected(t *testing.T) {
 // leaves a half-written snap.bin visible to readers.
 func TestSnapshotAtomicSwap(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]()
-	mustAppend := func(w *SnapshotWriter[Projection], sku string, price int64) {
+	w := NewSnapshotWriter[tProj]()
+	mustAppend := func(w *SnapshotWriter[tProj], sku string, price int64) {
 		t.Helper()
 		if err := appendP(w, makeProjection(sku, price)); err != nil {
 			t.Fatalf("Append %s: %v", sku, err)
@@ -223,13 +267,13 @@ func TestSnapshotAtomicSwap(t *testing.T) {
 	}
 
 	// replace
-	w2 := NewSnapshotWriter[Projection]()
+	w2 := NewSnapshotWriter[tProj]()
 	mustAppend(w2, "ZZZ", 200)
 	if _, err := w2.Write(dir, "snap.bin"); err != nil {
 		t.Fatal(err)
 	}
 
-	r, err := ReadSnapshotFile[Projection](final)
+	r, err := ReadSnapshotFile[tProj](final)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -251,7 +295,7 @@ func TestSnapshotRejectBadInput(t *testing.T) {
 	if err := os2WriteFile(path, []byte{}, 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotTruncated) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotTruncated) {
 		t.Errorf("empty file rejected with %v, want ErrSnapshotTruncated", err)
 	}
 
@@ -265,7 +309,7 @@ func TestSnapshotRejectBadInput(t *testing.T) {
 	if err := os2WriteFile(path, wrongMagic, 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotMagic) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotMagic) {
 		t.Errorf("wrong magic rejected with %v, want ErrSnapshotMagic", err)
 	}
 
@@ -274,7 +318,7 @@ func TestSnapshotRejectBadInput(t *testing.T) {
 	if err := os2WriteFile(path, buildValidHeader(5), 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotTruncated) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotTruncated) {
 		t.Errorf("truncated snapshot rejected with %v, want ErrSnapshotTruncated", err)
 	}
 
@@ -284,7 +328,7 @@ func TestSnapshotRejectBadInput(t *testing.T) {
 	if err := os2WriteFile(path, tampered, 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotBadBlob) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotBadBlob) {
 		t.Errorf("tampered-blob rejected with %v, want ErrSnapshotBadBlob", err)
 	}
 }
@@ -292,11 +336,11 @@ func TestSnapshotRejectBadInput(t *testing.T) {
 // TestSnapshotEmpty — empty (count=0) snapshot is valid; Get returns false.
 func TestSnapshotEmpty(t *testing.T) {
 	dir := t.TempDir()
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	if _, err := w.Write(dir, "empty.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r, err := ReadSnapshotFile[Projection](filepath.Join(dir, "empty.bin"))
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "empty.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -309,49 +353,48 @@ func TestSnapshotEmpty(t *testing.T) {
 	}
 }
 
-// TestSnapshotSKU32 — SKU exactly 32 chars (the boundary case) must succeed.
+// TestSnapshotSKU32 — key exactly 32 chars (the boundary case) must succeed.
 func TestSnapshotSKU32(t *testing.T) {
 	dir := t.TempDir()
 	sku := "SKU-" + repeat32("X", 32-4) // 4 + 28 = 32 chars
 	if len(sku) != 32 {
 		t.Fatalf("test bug: sku length %d", len(sku))
 	}
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	if err := appendP(w, makeProjection(sku, 1)); err != nil {
-		t.Fatalf("Append 32-char SKU: %v", err)
+		t.Fatalf("Append 32-char key: %v", err)
 	}
 	if _, err := w.Write(dir, "snap.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r, err := ReadSnapshotFile[Projection](filepath.Join(dir, "snap.bin"))
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "snap.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer r.Close()
 	if _, ok := r.Get(sku); !ok {
-		t.Error("32-char SKU Get returned false")
+		t.Error("32-char key Get returned false")
 	}
 }
 
 // TestSnapshotSKU33Rejected — one char over should error at Append.
 func TestSnapshotSKU33Rejected(t *testing.T) {
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	sku := repeat32("X", 33)
 	err := appendP(w, makeProjection(sku, 1))
 	if !errors.Is(err, ErrSKUTooLong) {
-		t.Errorf("33-char SKU Append: got %v want ErrSKUTooLong", err)
+		t.Errorf("33-char key Append: got %v want ErrSKUTooLong", err)
 	}
 }
 
-// TestSnapshotGetSpeed — probe thousands of random SKU lookups to give a
-// sense of the binary-search cost (not a benchmark per se).
+// TestSnapshotGetSpeed — probe thousands of random key lookups.
 func TestSnapshotGetSpeed(t *testing.T) {
 	if testing.Short() {
 		t.Skip("short mode")
 	}
 	dir := t.TempDir()
 	const n = 5000
-	w := NewSnapshotWriter[Projection]()
+	w := NewSnapshotWriter[tProj]()
 	skus := make([]string, n)
 	for i := 0; i < n; i++ {
 		skus[i] = fmt.Sprintf("Z%06d", i)
@@ -362,7 +405,7 @@ func TestSnapshotGetSpeed(t *testing.T) {
 	if _, err := w.Write(dir, "snap.bin"); err != nil {
 		t.Fatal(err)
 	}
-	r, err := ReadSnapshotFile[Projection](filepath.Join(dir, "snap.bin"))
+	r, err := ReadSnapshotFile[tProj](filepath.Join(dir, "snap.bin"))
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -429,20 +472,7 @@ func TestSnapshotRejectBadVersion(t *testing.T) {
 	if err := os2WriteFile(path, bs, 0o644); err != nil {
 		t.Fatal(err)
 	}
-	if _, err := ReadSnapshotFile[Projection](path); !errors.Is(err, ErrSnapshotVersion) {
+	if _, err := ReadSnapshotFile[tProj](path); !errors.Is(err, ErrSnapshotVersion) {
 		t.Errorf("v2-not-supported header rejected with %v, want ErrSnapshotVersion", err)
 	}
 }
-
-// makeProjection helper repeated here for the new tests.
-func makeProjection(sku string, price int64) Projection {
-	return Projection{
-		ID:          "sku:" + sku,
-		SKU:         sku,
-		Title:       "Item " + sku,
-		PriceIncVat: money.Cents(price),
-		Currency:    "SEK",
-		TaxClass:    "standard",
-		UpdatedAt:   time.Now().UnixMilli(),
-	}
-}
diff --git a/catalog/snapshot_unix.go b/projection/snapshot_unix.go
similarity index 85%
rename from catalog/snapshot_unix.go
rename to projection/snapshot_unix.go
index 7fb0e04..a5b4df1 100644
--- a/catalog/snapshot_unix.go
+++ b/projection/snapshot_unix.go
@@ -3,9 +3,9 @@
 // mmap-on-unix loader for SnapshotReader. The file handle stays alive for
 // the lifetime of the mmap region (the kernel needs it pinned). Closer
 // munmaps AND closes the fd. Uses MADV_RANDOM because the dominant access
-// pattern is binary-search-by-SKU — touching random record slots, not
+// pattern is binary-search-by-key — touching random record slots, not
 // streaming the file.
-package catalog
+package projection
 
 import (
 	"fmt"
@@ -39,10 +39,10 @@ func loadSnapshotBytes(path string) (data []byte, closer func() error, err error
 	data, mmapErr := unix.Mmap(int(f.Fd()), 0, int(size), unix.PROT_READ, unix.MAP_SHARED)
 	if mmapErr != nil {
 		_ = f.Close()
-		return nil, nil, fmt.Errorf("catalog: mmap %s: %w", path, mmapErr)
+		return nil, nil, fmt.Errorf("projection: mmap %s: %w", path, mmapErr)
 	}
 
-	// Binary-search by SKU = random page access; MADV_RANDOM tells the
+	// Binary-search by key = random page access; MADV_RANDOM tells the
 	// kernel NOT to do read-ahead — saves I/O on cold layers.
 	if maErr := unix.Madvise(data, unix.MADV_RANDOM); maErr != nil {
 		// non-fatal; degrade gracefully
diff --git a/projection/store.go b/projection/store.go
new file mode 100644
index 0000000..45bcfbe
--- /dev/null
+++ b/projection/store.go
@@ -0,0 +1,346 @@
+package projection
+
+// Store — the consumer half of the projection seam. A service feeds it the
+// snapshot/delta frames it receives off the bus and reads facts back by key.
+// The store owns a LOCAL read-only projection:
+//
+//   - A full snapshot stream (begin → chunk* → end, one MetaEpoch) is assembled
+//     into the store's OWN .bin (SnapshotWriter), atomically swapped in as the
+//     mmap base (SnapshotHolder). The base is the cold-start substrate — it
+//     survives a restart (reopened from disk), so the consumer serves reads
+//     immediately without waiting for a bus replay.
+//   - Per-batch deltas of the SAME epoch are applied to an in-memory overlay on
+//     top of the base (overlay wins; a delete leaves a tombstone that shadows the
+//     base and the fallback). The next snapshot folds the overlay into a new base.
+//
+// Read order on Get: overlay → mmap base → optional caller fallback. Scan walks
+// overlay + base (overlay wins, tombstones hidden) for enumeration / ranked
+// output.
+//
+// The store is generic over the WIRE value W (what rides on the bus, decoded by
+// the supplied Decoder) and the STORED value T (what the Transform projects W
+// down to, held in the .bin). For an identity consumer W == T with an identity
+// transform; catalog uses W = catalog.Projection.
+//
+// No shared volume: each consumer builds its own base from the bus. Concurrency:
+// HandleFrame is single-writer (drive it from one delivery goroutine); Get/Scan
+// are safe from many readers.
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"sync"
+)
+
+// Store is a consumer's local, cold-start-ready projection cache of T, fed from
+// a bus stream of wire values W.
+type Store[W, T any] struct {
+	dir, name string
+	decode    Decoder[W]
+	transform Transform[W, T]
+	fallback  Fallback[W]
+	holder    *SnapshotHolder[T] // mmap base; atomic-swap + drain-grace
+
+	mu      sync.RWMutex
+	epoch   int64                      // epoch of the live base
+	overlay map[string]overlayEntry[T] // same-epoch deltas layered on the base
+
+	// in-progress snapshot assembly (one epoch at a time)
+	building      *SnapshotWriter[T]
+	buildingEpoch int64
+	buildingSeen  int
+}
+
+type overlayEntry[T any] struct {
+	val     T
+	deleted bool
+}
+
+// NewStore opens a store backed by dir/name (its own .bin). decode and
+// transform are REQUIRED: decode parses the domain wire schema from a frame
+// payload; transform projects each wire value to the stored shape. If a base
+// from a previous run is present it is reopened immediately (cold-start ready);
+// its epoch is read from the sidecar so same-epoch deltas can resume. fallback
+// may be nil. Close() when done.
+func NewStore[W, T any](dir, name string, decode Decoder[W], transform Transform[W, T], fallback Fallback[W]) (*Store[W, T], error) {
+	if dir == "" || name == "" {
+		return nil, fmt.Errorf("projection: Store needs dir and name")
+	}
+	if decode == nil {
+		return nil, fmt.Errorf("projection: Store needs a decoder (declare the domain wire schema)")
+	}
+	if transform == nil {
+		return nil, fmt.Errorf("projection: Store needs a transform (declare which facts this consumer stores)")
+	}
+	s := &Store[W, T]{
+		dir:       dir,
+		name:      name,
+		decode:    decode,
+		transform: transform,
+		fallback:  fallback,
+		holder:    NewSnapshotHolder[T](0),
+		overlay:   make(map[string]overlayEntry[T]),
+	}
+	path := filepath.Join(dir, name)
+	if err := s.holder.Open(path); err == nil {
+		s.epoch = readEpochSidecar(path)
+		log.Printf("projection: Store reopened base %s (epoch=%d, %d entries)", path, s.epoch, s.holder.Len())
+	} else if !os.IsNotExist(err) {
+		// A corrupt/partial base on disk shouldn't crash boot — start empty and
+		// wait for the next snapshot; reads fall back meanwhile.
+		log.Printf("projection: Store ignoring unreadable base %s: %v", path, err)
+	}
+	return s, nil
+}
+
+// HandleFrame applies one projection event (snapshot frame or delta), routed by
+// its Meta. Drive this from a single delivery goroutine. A returned error means
+// the payload was undecodable (the caller may NACK); a frame that simply
+// doesn't apply (stale epoch, wrong assembly state) returns nil so the delivery
+// is acked.
+func (s *Store[W, T]) HandleFrame(meta map[string]string, payload []byte) error {
+	epoch, _ := strconv.ParseInt(meta[MetaEpoch], 10, 64)
+	switch meta[MetaSnapshotPhase] {
+	case SnapshotBegin:
+		return s.beginSnapshot(epoch, meta[MetaTenant])
+	case SnapshotChunk:
+		return s.chunkSnapshot(epoch, payload)
+	case SnapshotEnd:
+		return s.endSnapshot(epoch, meta)
+	case "":
+		return s.applyDelta(epoch, payload)
+	default:
+		return nil // unknown phase — ignore forward-compatibly
+	}
+}
+
+func (s *Store[W, T]) beginSnapshot(epoch int64, tenant string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if epoch <= s.epoch {
+		return nil // we already hold this base or newer
+	}
+	// Start (or restart) assembly for this epoch. A new begin supersedes any
+	// half-built snapshot (e.g. a producer that aborted mid-stream).
+	s.building = NewSnapshotWriter[T]().WithTenant(tenant)
+	s.buildingEpoch = epoch
+	s.buildingSeen = 0
+	return nil
+}
+
+func (s *Store[W, T]) chunkSnapshot(epoch int64, payload []byte) error {
+	ups, err := s.decode(payload)
+	if err != nil {
+		return fmt.Errorf("projection: decode snapshot chunk: %w", err)
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.building == nil || epoch != s.buildingEpoch {
+		return nil // not assembling this epoch
+	}
+	for _, u := range ups {
+		if u.Deleted || u.Key == "" {
+			continue // a snapshot carries live entries only
+		}
+		// Transform applied here — the base stores the consumer's shape.
+		if err := s.building.Append(u.Key, s.transform(u.Value)); err != nil {
+			log.Printf("projection: Store skip %q in snapshot: %v", u.Key, err)
+			continue
+		}
+		s.buildingSeen++
+	}
+	return nil
+}
+
+func (s *Store[W, T]) endSnapshot(epoch int64, meta map[string]string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.building == nil || epoch != s.buildingEpoch {
+		return nil
+	}
+	w := s.building
+	s.building = nil
+
+	// Completeness guard: if begin/end advertised a count and we appended fewer
+	// (a lost chunk), DON'T swap — keep the old base and wait for the next
+	// snapshot. Serving an incomplete projection silently is worse than stale.
+	if cs := meta[MetaCount]; cs != "" {
+		if want, perr := strconv.Atoi(cs); perr == nil && want != s.buildingSeen {
+			log.Printf("projection: Store snapshot epoch=%d incomplete (%d/%d) — discarding, keeping base epoch=%d",
+				epoch, s.buildingSeen, want, s.epoch)
+			return nil
+		}
+	}
+
+	path, err := w.Write(s.dir, s.name)
+	if err != nil {
+		return fmt.Errorf("projection: write snapshot base: %w", err)
+	}
+	// Persist the epoch sidecar (atomic) BEFORE swapping the in-memory reader, so
+	// a reopened store's epoch always matches the .bin already on disk (w.Write
+	// renamed the new file into place above). Writing it after Reload would leave
+	// a crash window where the new .bin carries a stale/absent epoch.
+	writeEpochSidecar(path, epoch)
+	if err := s.holder.Reload(path); err != nil {
+		return fmt.Errorf("projection: reload snapshot base: %w", err)
+	}
+	s.epoch = epoch
+	s.overlay = make(map[string]overlayEntry[T]) // base now includes everything ≤ epoch
+	log.Printf("projection: Store snapshot applied — base swapped epoch=%d (%d entries)", epoch, s.buildingSeen)
+	return nil
+}
+
+func (s *Store[W, T]) applyDelta(epoch int64, payload []byte) error {
+	ups, err := s.decode(payload)
+	if err != nil {
+		return fmt.Errorf("projection: decode delta: %w", err)
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	switch {
+	case s.building != nil:
+		// Under FIFO delivery a delta won't interleave a snapshot stream; if it
+		// does, the snapshot is authoritative for its epoch, so drop the delta.
+		return nil
+	case epoch < s.epoch:
+		return nil // stale — predates our base
+	case epoch > s.epoch:
+		// CONTRACT: a future-epoch delta is DROPPED, not buffered. Under FIFO
+		// per-queue delivery begin(N) always precedes delta(N), so this only
+		// happens after a reconnect that missed the whole snapshot — which a
+		// buffer couldn't reassemble anyway. Recovery is the next snapshot of
+		// this epoch; the consumer's fallback covers reads until then.
+		return nil
+	}
+	var upserts, deletes int
+	for _, u := range ups {
+		if u.Key == "" {
+			continue
+		}
+		if u.Deleted {
+			s.overlay[u.Key] = overlayEntry[T]{deleted: true}
+			deletes++
+			continue
+		}
+		// Transform applied here too — overlay holds the consumer's shape.
+		s.overlay[u.Key] = overlayEntry[T]{val: s.transform(u.Value)}
+		upserts++
+	}
+	if upserts+deletes > 0 {
+		log.Printf("projection: Store delta applied epoch=%d (+%d/-%d, overlay=%d)",
+			epoch, upserts, deletes, len(s.overlay))
+	}
+	return nil
+}
+
+// Get resolves a value by key: overlay (incl. tombstone) → mmap base →
+// fallback. A tombstoned key returns (zero, false) and never reaches the
+// fallback. A fallback hit is run through the same Transform.
+func (s *Store[W, T]) Get(sku string) (T, bool) {
+	var zero T
+	s.mu.RLock()
+	e, ok := s.overlay[sku]
+	s.mu.RUnlock()
+	if ok {
+		if e.deleted {
+			return zero, false
+		}
+		return e.val, true
+	}
+	if v, ok := s.holder.Get(sku); ok {
+		return v, true
+	}
+	if s.fallback != nil {
+		if w, ok := s.fallback(sku); ok {
+			return s.transform(w), true
+		}
+	}
+	return zero, false
+}
+
+// Scan walks every live (key, value) — overlay first (overlay wins; tombstones
+// hidden), then the mmap base minus any key the overlay already covered. Return
+// false from fn to stop early. The fallback is NOT consulted (it resolves
+// single misses, not enumeration). Callers that need top-N-by-value collect and
+// sort in memory — on-disk order is by key. Held under RLock for the whole
+// walk, so it blocks a concurrent snapshot swap; keep fn cheap.
+func (s *Store[W, T]) Scan(fn func(key string, v T) bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	seen := make(map[string]struct{}, len(s.overlay))
+	for k, e := range s.overlay {
+		seen[k] = struct{}{}
+		if e.deleted {
+			continue
+		}
+		if !fn(k, e.val) {
+			return
+		}
+	}
+	s.holder.Scan(func(k string, v T) bool {
+		if _, ok := seen[k]; ok {
+			return true // overlay already emitted (or tombstoned) this key
+		}
+		return fn(k, v)
+	})
+}
+
+// IsDeleted reports whether the overlay holds a tombstone for key — a delete
+// seen on the bus since the current base.
+func (s *Store[W, T]) IsDeleted(sku string) bool {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	e, ok := s.overlay[sku]
+	return ok && e.deleted
+}
+
+// Epoch returns the epoch of the current base (0 if none yet).
+func (s *Store[W, T]) Epoch() int64 {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.epoch
+}
+
+// Stats returns observability counters (base size, overlay size, epoch).
+func (s *Store[W, T]) Stats() (epoch int64, baseCount, overlayCount int) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	return s.epoch, s.holder.Len(), len(s.overlay)
+}
+
+// Close releases the mmap base.
+func (s *Store[W, T]) Close() error { return s.holder.Close() }
+
+// ── epoch sidecar ────────────────────────────────────────────────────────────
+// The .bin header has no room for the epoch, so it's persisted next to it as
+// "<name>.epoch" — read on boot so a reopened base can resume same-epoch deltas.
+
+func epochSidecarPath(binPath string) string { return binPath + ".epoch" }
+
+// writeEpochSidecar writes the epoch atomically (temp+rename) so a crash mid-write
+// can't leave a torn file that parses as a bogus epoch.
+func writeEpochSidecar(binPath string, epoch int64) {
+	p := epochSidecarPath(binPath)
+	tmp := p + ".tmp"
+	if err := os.WriteFile(tmp, []byte(strconv.FormatInt(epoch, 10)), 0o644); err != nil {
+		log.Printf("projection: Store epoch sidecar write: %v", err)
+		return
+	}
+	if err := os.Rename(tmp, p); err != nil {
+		log.Printf("projection: Store epoch sidecar rename: %v", err)
+		_ = os.Remove(tmp)
+	}
+}
+
+func readEpochSidecar(binPath string) int64 {
+	b, err := os.ReadFile(epochSidecarPath(binPath))
+	if err != nil {
+		return 0
+	}
+	n, _ := strconv.ParseInt(strings.TrimSpace(string(b)), 10, 64)
+	return n
+}
diff --git a/shipping/shipping.go b/shipping/shipping.go
new file mode 100644
index 0000000..2c4b08e
--- /dev/null
+++ b/shipping/shipping.go
@@ -0,0 +1,140 @@
+// Package shipping is the platform's shared shipping seam: the cross-module
+// vocabulary for delivery options and the Provider interface. It holds only the
+// wire value types plus the interface, so the producer (a shipping adapter like
+// PostNord) and every consumer (checkout, cart, backoffice) agree on the types.
+// It imports only the stdlib — never an app module, never a transport library.
+//
+// Dependency rule: packages under platform/ import only third-party libraries
+// and other platform/ packages — never an application module. Application
+// modules depend downward onto platform. This is what keeps it a stable seam.
+//
+// See platform/inventory for the sibling pattern (value types + interface in
+// one stdlib-only package, test adapter in _test.go).
+package shipping
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+)
+
+// ── Provider interface ─────────────────────────────────────────────────────
+
+// Provider is the cross-module seam for shipping delivery options. Checkout
+// calls GetShippingOptions to present available delivery methods to the
+// customer. Every implementation behind this interface is free to choose its
+// internal provider (PostNord, DHL, Bring, etc.) — the consumer never needs
+// to know.
+//
+// This is the policy-first seam: shipping selection policy lives here; the
+// provider-specific HTTP/REST details live behind it.
+type Provider interface {
+	// GetShippingOptions returns available delivery options for a recipient
+	// address. Returns an empty slice (not an error) when the provider has no
+	// matching options. Returns ErrNoOptions when no provider in a multi-provider
+	// setup could serve the address.
+	GetShippingOptions(ctx context.Context, req GetOptionsRequest) ([]DeliveryOptionGroup, error)
+}
+
+// ── Request types ─────────────────────────────────────────────────────────
+
+// GetOptionsRequest is what a caller provides to get shipping options.
+type GetOptionsRequest struct {
+	// Country is the ISO 3166-1 alpha-2 recipient country (e.g. "SE", "NO").
+	Country string `json:"country"`
+	// PostCode is the recipient postal code.
+	PostCode string `json:"post_code"`
+	// Locale is the IETF BCP 47 language tag for descriptions (e.g. "sv", "en").
+	// Empty means the provider chooses a default.
+	Locale string `json:"locale,omitempty"`
+	// Items optionally describes the cart items for dimension/weight-based
+	// filtering. May be nil or empty for providers that don't need item data.
+	Items []OptionItem `json:"items,omitempty"`
+}
+
+// OptionItem describes one line item for shipping calculations (weight, volume,
+// category-based restrictions).
+type OptionItem struct {
+	SKU      string `json:"sku"`
+	Quantity int    `json:"quantity"`
+	// WeightGrams is the item weight in grams, if known.
+	WeightGrams int `json:"weight_grams,omitempty"`
+}
+
+// ── Response types ────────────────────────────────────────────────────────
+
+// DeliveryOptionGroup groups shipping options by fulfilment method (e.g. home
+// delivery, pickup point, parcel locker). A provider returns one or more
+// groups; each group has one default option and optional alternatives (e.g.
+// different pickup point locations).
+type DeliveryOptionGroup struct {
+	// Type is the fulfilment-method category: "home", "pickup", "locker",
+	// "mailbox", etc.
+	Type string `json:"type"`
+	// Name is the display name for the group (e.g. "Home delivery",
+	// "Pickup point"). Provider-localised to the request Locale.
+	Name string `json:"name"`
+	// DefaultOption is the recommended option within this group.
+	DefaultOption DeliveryOption `json:"default_option"`
+	// AdditionalOptions are alternatives within the same group (e.g. other
+	// pickup point locations, different time slots).
+	AdditionalOptions []DeliveryOption `json:"additional_options,omitempty"`
+}
+
+// DeliveryOption is one selectable shipping method or service point.
+type DeliveryOption struct {
+	// ID is the provider-scoped unique identifier for this option. Used when
+	// the caller confirms the selection (e.g. SetCartDelivery).
+	ID string `json:"id"`
+	// Name is a short display name (e.g. "Home delivery", "PostNord Storgatan").
+	Name string `json:"name"`
+	// Description is a longer description for the checkout UI.
+	Description string `json:"description,omitempty"`
+	// DeliveryDays is a human-readable estimate (e.g. "2-3 working days",
+	// "1-2 days"). Provider-localised.
+	DeliveryDays string `json:"delivery_days,omitempty"`
+	// PickupLocation is set when this option is a physical pickup point.
+	PickupLocation *PickupLocation `json:"pickup_location,omitempty"`
+}
+
+// PickupLocation describes a physical pickup point (store, locker, post office).
+type PickupLocation struct {
+	ID      string  `json:"id"`
+	Name    string  `json:"name"`
+	Address Address `json:"address"`
+}
+
+// Address is a simplified postal address for delivery or pickup.
+type Address struct {
+	Street     string `json:"street"`
+	Street2    string `json:"street2,omitempty"`
+	City       string `json:"city"`
+	PostCode   string `json:"post_code"`
+	Country    string `json:"country"` // ISO 3166-1 alpha-2
+}
+
+// ── Sentinels ─────────────────────────────────────────────────────────────
+
+// ErrNoOptions is returned by a multi-provider orchestrator when no registered
+// provider could serve the given address. A single provider returns an empty
+// slice (not this error) for an unsupported address.
+var ErrNoOptions = errors.New("shipping: no delivery options available")
+
+// ErrProviderUnavailable is returned when a provider's API is unreachable or
+// returns a non-retryable error. The caller may fall back to another provider
+// or degrade gracefully.
+var ErrProviderUnavailable = errors.New("shipping: provider unavailable")
+
+// ── Wire helpers ──────────────────────────────────────────────────────────
+
+// Encode serialises a DeliveryOptionGroup slice for caching or event payloads.
+func Encode(groups []DeliveryOptionGroup) ([]byte, error) {
+	return json.Marshal(groups)
+}
+
+// Decode parses a DeliveryOptionGroup slice from Encode output.
+func Decode(raw []byte) ([]DeliveryOptionGroup, error) {
+	var out []DeliveryOptionGroup
+	err := json.Unmarshal(raw, &out)
+	return out, err
+}
diff --git a/shipping/shipping_test.go b/shipping/shipping_test.go
new file mode 100644
index 0000000..f65e45e
--- /dev/null
+++ b/shipping/shipping_test.go
@@ -0,0 +1,306 @@
+package shipping
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"testing"
+)
+
+// InMemoryProvider is a thread-safe in-memory implementation of Provider for
+// tests — no external API calls needed. Backed by a map from
+// "country|postcode" to available delivery option groups.
+type InMemoryProvider struct {
+	mu   sync.Mutex
+	opts map[string][]DeliveryOptionGroup // key: "country|postcode"
+	err  error                            // injected error for testing failure paths
+}
+
+// NewInMemoryProvider creates a provider seeded with delivery option groups.
+// seed is a sequence of (country, postcode, []DeliveryOptionGroup) triples.
+func NewInMemoryProvider(seed ...any) *InMemoryProvider {
+	p := &InMemoryProvider{
+		opts: make(map[string][]DeliveryOptionGroup),
+	}
+	for i := 0; i+2 < len(seed); i += 3 {
+		country, ok := seed[i].(string)
+		if !ok {
+			continue
+		}
+		postcode, ok := seed[i+1].(string)
+		if !ok {
+			continue
+		}
+		groups, ok := seed[i+2].([]DeliveryOptionGroup)
+		if !ok {
+			continue
+		}
+		p.opts[providerKey(country, postcode)] = groups
+	}
+	return p
+}
+
+func providerKey(country, postcode string) string {
+	return country + "|" + postcode
+}
+
+// SetError injects an error that GetShippingOptions returns on every call
+// (for testing failure paths). Nil clears the error.
+func (p *InMemoryProvider) SetError(err error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	p.err = err
+}
+
+func (p *InMemoryProvider) GetShippingOptions(_ context.Context, req GetOptionsRequest) ([]DeliveryOptionGroup, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	if p.err != nil {
+		return nil, p.err
+	}
+
+	groups, ok := p.opts[providerKey(req.Country, req.PostCode)]
+	if !ok {
+		return []DeliveryOptionGroup{}, nil
+	}
+	return groups, nil
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────────
+
+func TestProviderReturnsOptions(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryProvider(
+		"SE", "17173", []DeliveryOptionGroup{
+			{
+				Type: "home",
+				Name: "Home delivery",
+				DefaultOption: DeliveryOption{
+					ID:           "home-1",
+					Name:         "Home delivery",
+					Description:  "Delivered to your door",
+					DeliveryDays: "2-3 working days",
+				},
+			},
+			{
+				Type: "pickup",
+				Name: "Pickup point",
+				DefaultOption: DeliveryOption{
+					ID:           "pickup-1",
+					Name:         "PostNord Storgatan",
+					Description:  "Collect at PostNord Storgatan",
+					DeliveryDays: "2-3 working days",
+					PickupLocation: &PickupLocation{
+						ID:   "loc-1",
+						Name: "PostNord Storgatan",
+						Address: Address{
+							Street:   "Storgatan 9",
+							City:     "Solna",
+							PostCode: "17173",
+							Country:  "SE",
+						},
+					},
+				},
+			},
+		},
+	)
+
+	groups, err := p.GetShippingOptions(ctx, GetOptionsRequest{
+		Country:  "SE",
+		PostCode: "17173",
+		Locale:   "sv",
+	})
+	if err != nil {
+		t.Fatalf("GetShippingOptions: %v", err)
+	}
+	if len(groups) != 2 {
+		t.Fatalf("expected 2 option groups, got %d", len(groups))
+	}
+
+	// Verify home delivery option
+	if groups[0].Type != "home" {
+		t.Fatalf("first group: expected type=home, got %q", groups[0].Type)
+	}
+	if groups[0].DefaultOption.ID != "home-1" {
+		t.Fatalf("home default option: expected ID=home-1, got %q", groups[0].DefaultOption.ID)
+	}
+
+	// Verify pickup option with location
+	if groups[1].Type != "pickup" {
+		t.Fatalf("second group: expected type=pickup, got %q", groups[1].Type)
+	}
+	loc := groups[1].DefaultOption.PickupLocation
+	if loc == nil {
+		t.Fatal("pickup option: expected a PickupLocation, got nil")
+	}
+	if loc.Address.City != "Solna" {
+		t.Fatalf("pickup address city: expected Solna, got %q", loc.Address.City)
+	}
+}
+
+func TestProviderUnsupportedAddress(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryProvider() // no seed data
+
+	groups, err := p.GetShippingOptions(ctx, GetOptionsRequest{
+		Country:  "NO",
+		PostCode: "0150",
+	})
+	if err != nil {
+		t.Fatalf("expected nil error for unsupported address, got %v", err)
+	}
+	if groups == nil {
+		t.Fatal("expected non-nil empty slice for unsupported address, got nil")
+	}
+	if len(groups) != 0 {
+		t.Fatalf("expected 0 option groups for unsupported address, got %d", len(groups))
+	}
+}
+
+func TestProviderError(t *testing.T) {
+	ctx := context.Background()
+	p := NewInMemoryProvider(
+		"SE", "17173", []DeliveryOptionGroup{
+			{
+				Type: "home",
+				Name: "Home delivery",
+				DefaultOption: DeliveryOption{
+					ID:   "home-1",
+					Name: "Home delivery",
+				},
+			},
+		},
+	)
+
+	injected := errors.New("api timeout")
+	p.SetError(injected)
+
+	_, err := p.GetShippingOptions(ctx, GetOptionsRequest{
+		Country:  "SE",
+		PostCode: "17173",
+	})
+	if err == nil {
+		t.Fatal("expected injected error, got nil")
+	}
+	if !errors.Is(err, injected) {
+		t.Fatalf("expected injected error %v, got %v", injected, err)
+	}
+}
+
+func TestProviderAdditionalOptions(t *testing.T) {
+	ctx := context.Background()
+
+	additionalPoints := make([]DeliveryOption, 2)
+	additionalPoints[0] = DeliveryOption{
+		ID:   "pickup-2",
+		Name: "ICA Maxi Solna",
+		PickupLocation: &PickupLocation{
+			ID:   "loc-2",
+			Name: "ICA Maxi Solna",
+			Address: Address{
+				Street:   "Solna Centrum 5",
+				City:     "Solna",
+				PostCode: "17173",
+				Country:  "SE",
+			},
+		},
+	}
+	additionalPoints[1] = DeliveryOption{
+		ID:   "pickup-3",
+		Name: "Coop Solna",
+		PickupLocation: &PickupLocation{
+			ID:   "loc-3",
+			Name: "Coop Solna",
+			Address: Address{
+				Street:   "Solna Torg 2",
+				City:     "Solna",
+				PostCode: "17173",
+				Country:  "SE",
+			},
+		},
+	}
+
+	p := NewInMemoryProvider(
+		"SE", "17173", []DeliveryOptionGroup{
+			{
+				Type: "pickup",
+				Name: "Pickup point",
+				DefaultOption: DeliveryOption{
+					ID:   "pickup-1",
+					Name: "PostNord Storgatan",
+				},
+				AdditionalOptions: additionalPoints,
+			},
+		},
+	)
+
+	groups, err := p.GetShippingOptions(ctx, GetOptionsRequest{
+		Country:  "SE",
+		PostCode: "17173",
+	})
+	if err != nil {
+		t.Fatalf("GetShippingOptions: %v", err)
+	}
+	if len(groups) != 1 {
+		t.Fatalf("expected 1 option group, got %d", len(groups))
+	}
+
+	addl := groups[0].AdditionalOptions
+	if len(addl) != 2 {
+		t.Fatalf("expected 2 additional options, got %d", len(addl))
+	}
+	if addl[0].ID != "pickup-2" {
+		t.Fatalf("additional[0]: expected ID=pickup-2, got %q", addl[0].ID)
+	}
+}
+
+func TestRoundTripEncodeDecode(t *testing.T) {
+	original := []DeliveryOptionGroup{
+		{
+			Type: "home",
+			Name: "Home delivery",
+			DefaultOption: DeliveryOption{
+				ID:           "home-1",
+				Name:         "Home delivery",
+				DeliveryDays: "2-3 working days",
+			},
+		},
+	}
+
+	encoded, err := Encode(original)
+	if err != nil {
+		t.Fatalf("Encode: %v", err)
+	}
+
+	decoded, err := Decode(encoded)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+
+	if len(decoded) != 1 {
+		t.Fatalf("round-trip: expected 1 group, got %d", len(decoded))
+	}
+	if decoded[0].DefaultOption.ID != "home-1" {
+		t.Fatalf("round-trip: expected ID=home-1, got %q", decoded[0].DefaultOption.ID)
+	}
+}
+
+func TestNoOptionsSentinelNotReturnedBySingleProvider(t *testing.T) {
+	// ErrNoOptions is for multi-provider orchestrators. A single provider
+// returns an empty slice for unsupported addresses. Verify that the
+// in-memory provider follows this contract.
+	ctx := context.Background()
+	p := NewInMemoryProvider()
+
+	groups, err := p.GetShippingOptions(ctx, GetOptionsRequest{
+		Country:  "FI",
+		PostCode: "00100",
+	})
+	if err != nil {
+		t.Fatalf("expected nil for unsupported address, got %v", err)
+	}
+	if groups == nil {
+		t.Fatal("expected non-nil empty slice, got nil")
+	}
+}